there's a pretty obvious reason for that.
most democracies will naturally fall into a system where there is at least a semblance of balance between left and right wing parties. that is because they are all competing to win and operating left or right the centre ground for any particular given country give the parties their best chance of success.
in Wales we don't have a political system that operates in isolation. it is very intertwined with that of the rest of the UK.
if we were independent then you would probably find that the tory party would shift considerably to the left in Wales to become competitive, based on the demographics of the Welsh population.
they cannot do that as they would be going against the Westminster Tory party l, who do not want to lurch to the left, as they have a large right wing support base in England.
this hamstrings the opposition in Wales, and means that labour will have a much easier ride here.