The Welsh employed the longbow long before any of their conquerors and used to great effect against their enemies for centuries.

Bows and arrows have been around since Paleolithic times, with evidence of them as early as 8000-9000 BC in Germany.   http://www.newarchaeology.com/articles/history_bow_and_arrows.php

Kennewick man, the controversial skeleton found in the banks of the Columbia River inKennewick,Washington dates to roughly 7500 BC. A CT scan revealed a stone, projectile point embedded in his hip.

Oetzi the Iceman was found with a quiver of arrows with flint heads and an unfinished yew longbow–taller than he was–in his pack.  He dates to 3300 BC.

A new find in Norway revealed 4500 year old bows and arrows that are very similar in form and function to those found in the Yukon dating to the same time period.

The confirmed first use of the longbow was in 633 AD, in a battle between the Welsh, led by Cadwallon ap Cadfan of Gwynedd, against the Northumbrians.   http://www.themiddleages.net/longbow.html

The shot killed Ofrid (or Osric?), son of Edwin of Northumbria, who just happened to be Cadwallon’s foster-uncle.  Cadwallon had allied himself with Penda of Mercia in an attempt to drive the Northumbrians from Gwynedd. Cadwallon was successful.   http://www.earlybritishkingdoms.com/bios/cadwagd.html

Saxons, as a rule, were not archers.  It is another five centuries before there is any recorded use of a longbow in England.

The men of Wales used longbows against the Normans, from the moment the Normans crossed the English Channel to conquer England and Wales. One article states: “There is reliable evidence of Welsh archery eleven years before Hastings in the account of Ralph, Eorl of Hereford, on the expedition he led into Wales. When the Saxon horsemen had ridden into the Welsh mountains they were ambushed by archers who shot so accurately and strongly that, according to the Abingdon Chronicle, ‘the English people fled, before ever a spear had been thrown, because they were on horseback’. One estimate from the time puts the English casualties at five hundred whilst the Welsh suffered no losses. Here was a lesson that, if the Saxons had learned from it, could have changed the outcome of the Battle of Hastings; cavalry are helpless against well ordered archers. https://regia.org/research/warfare/SaxonArchery.htm

According to Gerald of Wales: ‘the bows these Welshmen use are not made of horn, or ivory, or yew but of wild elm, and not beautifully formed or polished, quite the opposite; they are rough and lumpy, but stout and strong nonetheless, not only able to shoot an arrow a long way, but also to inflict very severe wounds.’

And further: [I]n the war against the Welsh, one of the men of arms was struck by an arrow shot at him by a Welshman. It went right through his thigh, high up, where it was protected inside and outside the leg by his iron chausses, and then through the skirt of his leather tunic; next it penetrated that part of the saddle which is called the alva or seat; and finally it lodged in his horse, driving so deep that it killed the animal.^[3][4]

“This tells us much about the bows in use by the mid twelfth century, and probably those in use in the eleventh. The reference to horn and ivory show that composite bows were known, and the inclusion of yew shows they knew of this best of bow timbers. The most useful reference though is to the description of the Welsh bows as ‘rough and lumpy’; this is evidence that the Welsh bowyers understood the importance of ‘following the grain’ and leaving knots proud when making a reliable, powerful bow. It also shows that they knew that of the native timbers British elm was a superior timber to British yew for making bows (the best yew for bows came from Scandinavia, Spain and Austria). It is quite likely that these are the type of bows Eorl Ralph faced a hundred years earlier.” https://regia.org/research/warfare/SaxonArchery.htm

Starting 1252 in England, the longbow was finally accepted as a formal military weapon.  “In 1252 the Assize of Arms required that all landowning yeomen with an annual income between 40 to a 100 shillings were to be armed and trained with a longbow (war bow) and the more wealthy yeomen were also required to possess a sword, buckler, dagger and to be trained in their use.”   http://robinhode.webs.com/yeomen.html King Henry III had discovered yet again the power of the Welsh longbow uring his Welsh campaigns, after seeing how effectively the Welsh used the bow.

The English didn’t adopt the bow overnight, however, and didn’t take it into battle in the same way the  Welsh did. One of the greatest victories for Llywelyn ap Gruffydd, the last native ruler of Wales, was in 1257 at the Battle of Cymerau at which the Normans lost 3000 men (https://sarahwoodbury.com/cymerau/).  Twenty-five years later, in 1282 at Llandeilo Fawr, the Normans cowered for two days under a hail of arrows from the Welsh and suffered a great defeat.

Once the Welsh were defeated by King Edward, bowmen were recruited in large numbers into the English armies, as this was one of the few ways they could earn a living wage.

During the hundred years war with France predominantly longbowmen, both Welsh and, by now, English, were a prominent part of the English army, sometimes outnumbering the Men-at-Arms by as much as 10:1. The average was a ratio of about 3:1.”   http://www.archers.org/default.asp?section=History&page=longbow

What is it about the longbow that is both effective and also prevented its earlier adaptation?  This has to do with 1)  it’s size, and 2) the length of time required to learn its use.

The standard yew longbow was over 6 feet long (6 ft. 6 inches), with a yard long arrow.  They are powerful weapons that require enormous strength to draw.   In general, the draw weight is 120-150 pounds, with a range between 200 and 300 yards.  “In battle, longbow formations fired 10-12 volleys per minute. Each archer was provided 60-72 arrows. A force of 4,000 longbowmen could loose 240,000 arrows within the space of five minutes.”   http://www.militaryhistory.teamultimedia.com/History%20of%20Weapons/Welsh%20and%20English%20Longbow.html

Thus, in order to master its use, a man must practice.   A lot.  Once King Edward I realized the longbow’s full potential, he adopted it from the Welsh, such that “To ensure a steady stream of bowmen for his army, Edward I banned all sports except archery on Sundays. Shooting ranges were set up on or near church property so parishioners would follow worship services with archery practice.”    http://www.militaryhistory.teamultimedia.com/History%20of%20Weapons/Welsh%20and%20English%20Longbow.html

Edward III used the long bow to great effect during the Hundred Years War, filling his ranks with Welsh and English longbowmen that decimated the French ranks, particularly at the Battles of Crecy and Agincourt.   http://en.wikipedia.org/wiki/English_longbow