She was the reigning English monarch. And therein lies the problem. The fact is that 750 years ago the title of Prince of Wales was stolen by the English King Edward I after his troops killed the reigning Prince of Wales, Llywelyn ap Gruffydd in battle in 1282, and then executed Llywelyn's brother Daffydd ap Gruffydd in 1283, (thus making Daffydd the last Welsh-born ruler of that nation.)
A few years later Edward I named his 16-year-old son (who would go on to succeed him as king) the first Prince of Wales in 1301. And thus the English tradition began.
About 90 years later, Owain ap Thomas ap Rhodri, known in history books as Owain Lawgoch, a Welsh descendant of what had once been the chief Welsh royal family, tried an abortive attempt at taking control of the country, but with no success.
30 years later Owain ap Gruffydd, better known as Owain Glyn Dŵr, claimed the title of Prince of Wales and led a 15 year revolt against the British. He was defeated in battle twice by the English Prince Henry (who became King Henry the Fifth of Great Britain) but escaped and disappeared, apparently living out the remainder of his life in obscurity somewhere in the mountains of northern Welsh.
Because of the history listed above, Welsh nationalists --even today-- argue that the title of Prince of Wales was stolen from the Welsh and that only Welsh descendants of one of the original royal families of Wales merits the title.