1) There are more teams invested at a high level, so the EPL is a tougher league in which to buy all the best players. Bayern has little competition for the best players (Dortmund gets them with expertise more than $$$). Likewise PSG and to a lesser degree Juventus. No other league has 6 big spending clubs competing for the best players domestically.

2) other leagues do more to help their teams succeed internationally. The EPL far more often will cram a tough fixture list around the UCL and Europa games while other leagues are more likely to give a long break before the games. I don’t know if this is true generally or was true a couple times so fans remember it.

3) The greater number of strong clubs means there are fewer chances for clubs to rest their players for international play, or just relax during less competitive league games.

4) the EPL has a faster and more physical style of play, so their teams are more tired and beat up for international play.

Anyway, that’s what I hear. No one could argue effectively that the EPL has been more successful in international play during the Messi / Ronaldo era. Clearly La Liga has had more success lately.