The 900 pound gorilla in the corner of classical music is, of course, Wagner. It isn't his musical influence, which was enormous, that we're worrying about, but his recasting of opera into something that must be "significant." For him it was a tool for German nationalism, a concept that Wagner virtually invented. Prior to Wagner composers were content to be popular, but afterward they were required to be important as well.
Romantic style passed away, but the belief in significance is still very much with us. The problem is that composers are unable to distinguish between significant and pompous, or even more likely, between significant and boring, or even between significant and incomprehensible.
Alfano, perhaps the logical heir of Puccini, growing up in the midst of verismo, felt compelled to study in Germany instead of being content to be Italian. Dallapiccola imitated the distinctly German Second Viennese School and wrote twelve-tone music. No one came to carry on the purely Italian tradition.
Opera died because Italian music died, subsumed in the rage for things German, and no one has come to replace it.