If it had originated and stayed on continental Europe; it would more frequent there today.
not when all those other areas of Europe have a long history of being overrun and conquered by various other groups at many various times. As well as a much longer history of assimilation.
As I said earlier, Ireland was never overrun by any major conquering force historically so it remained isolated. As for Britain it was not truly overrun by the Romans until all other areas of western Europe, like Gaul, Iberia, had been fully romanized and assimilated. Britain was never fully romanized to the extent Gaul was. And since R1b-L21 is found in England at moderate to high levels, obviously the Saxons didnt commit genocide against the native Britons either.
as for R1b-L21 being found in western Europe, just because a population was once in a place in more larger numbers doesnt mean it stays that way forever.
There are virutally no Native AMericans left in the part of the U.S. I live in although at one time it was overrun with them.
population movements, wars, and various other factors change the demographic make up of an area if they go on for a long time and on a large scale.
Although the Isles experienced those things like the rest of western europe they didnt experience them on the same scale or for as long a time as other areas of western europe.
That makes sense as to why R1b-L21 is still found in the Isles at higher frequency today than anywhere else, especially those parts of the Isles with little outside intrusion
historically.
but again, you still have no explanation as to why R1b-L21 is found in other areas of Europe besides Britain. Yeah, its found at lower frequency but, again, how did it get there at all?
If it originated from England, (which it didn't), then how did it reach other areas of western europe? The frequency maybe low but the spread is wide in other parts of western europe, so what is the reason for that since there was no major historical migration from ancient Britain to various other areas of western europe?