Because the ability to delete posts is only possible with a centralized single source of truth database, with all the downsides that entails.

Nostr notes are not simple database records, instead they resemble Bitcoin transactions. They always have a public key and a digital signature, therefore they are tamper-proof and provably genuine.

While on Twitter posts are real insofar as they come from their servers (and so they can be deleted) in Nostr the notes are real all by themselves. Even if you managed to delete one of your notes from all relays any local copy on any client could be stored and revealed again at a later time, and its signature would still be valid and point to you.

That's my thinkboi take.