A lot of it comes down to historical time since Wales was integrated into Great Britain (16th century) versus Ireland (19th century) and Scotland (18th century). As a result, Scotland and Ireland are significantly less integrated with England than Wales. England pursued a very explicit policy of stamping out Welsh language and culture, and while they did the same in Ireland and to a lesser extent Scotland they had less time to do it. Both are larger than Wales in terms of population and both are further away from London.
Fair to ask why those movements are so much weaker than their counterparts in Scotland and especially Ireland, though.
I don't think they are in 2021. the Welsh nationalists might not poll as well as the SNP or Sinn Fein, but Plaid Cymru are still a sizeable force in Welsh politics, as is the call for independence even among Labour voters
they're much less racist than Bloc Quebecois for one. but all independence parties in the UK are regionalist representation with independence as a very long term goal. even Sinn Fein, who are by far the most radical with their ties to the IRA, sit in the northern Irish assembly although they boycott the main central British parliament
