Why are Celtic and Rangers not aloud to play in the English leagues but Cardiff and Swansea are?

There wasn't a professional football league in Wales at the time each club was founded, so they decided to join the closest one that was in existence. They're not the only Welsh clubs to play in the English league system: Wrexham and Newport Country have played in the Football League, and Merthyr Tydfil have reached the highest level in the non-league pyramid. The only downside to their participation in the English League is that they're not allowed to compete in the Welsh Cup, and so cannot qualify for Europe via that route.