1. because La Liga in general is on the weakest level since 2000s, so we are defending well against mostly very average to poor sides
2. a type of football in La Liga is latino-technical with less pressing, running and physique than in EPL or let's say Germany. So, even a defense with Busi can be stone-wall in that type of circumstances
3. a type of attacks in La liga is again also latino-technical with majority of actions coming from possession, passing, some sort of TikiTaka type of football.
While in CL, there are more crosses, headers and different sorts of attacks where Mats can't cope well with crosses, our defenders (plus Busi) can't cope with headers etc + paired with more physical and faster football compared to La Liga
A somewhat exaggerated analogy: imagine Saudi Arabia being a champion of Asia and playing a good defense, but then losing to a majority of European teams, Brazil and Argentina with 4:0.
And then their journos asking: I don't get it, our defense is stone-wall against Yemen and Turkmenistan, but we concede for fun against Messi, Haaland and Mbappe or France/Brazil/England. Why?