The short answer is that Scotland and N.Ireland already had notably different systems in place prior to 1997 so they inherited a more advanced set of powers post devolution.

It's basically one say traffic since then in terms of more and more powers being devolved from Westminster to Edinburgh, Cardiff and Belfast but Wales is still less far down that road and probably always will be.

The other element is that Wales & England are far more aligned economically in terms of cross-border travel etc which probably dampens the demand to do things differently all the time.