I think it's unfair to just use the Arena di Verona as the only example of a well-preserved example of Roman amphitheater across Europe, other examples where the outer facade and the inside are well preserved are notably in Arles and Nimes (France), or the one in Pola (Croatia), all well-preserved and all to this day used for artistic performances. As long as it's simply a two-storied, stocky building it's nothing too hard to weather the ages.

I think that when one thinks of ruined Roman amphitheaters one has to think about the Colosseum, which is the example par excellence of it. But in this case, the reason lies in the history it went through. Earthquakes notably in 443 AD and in 1349 AD caused the damage and the collapse on the sides, which were sitting on already unstable terrain, and it's no doubt that its constant repurposing (most notably as a fortress by the local Frangipani family in the Middle Ages) contributed to both defacing and weakening its structure.