maybe related to their mentality. PSG can play well and very strong in Ligue 1, even always dominating. They have high confidence to win the match in Ligue 1, but when at UCL and facing a team with good quality players, they play with pressure. the competition between each team in UCL will definitely be different when they compete in Ligue 1. This competition between teams that they may not get in the league. if Ligue 1 is more competitive, maybe PSG can also develop and start to get used to the pressure of the game like in UCL.
I don't think it's any news again that the competition in ligue 1 is such that doesn't encourages enough competition amongst the teams in the league such that they don't get to cope well beyond their domestic league and a typical example has been with PSG who stands to be onr of the top teams in the league but turns out weak out there when playing against top Europe teams away from the ligue 1 and so they have their confidence and dominance mostly in the French league more than they do anywhere. But then talking about developing that strength and spirit of been a strong contender, I feel PSG shohave developed it by now considering they have been so experienced in the UCL with their regular appearances.