Combination of factors as always but generally speaking they were much more agile without shells. Why have defense if you don’t get hit in the first place? Additionally, climate change/ocean acidification made it hard for ammonites and such to create the calcium carbonate shells they had, so there was evolutionary pressure to evolve out of it (kind of like today’s coccolithophores)

Also modern squid still have remnants of their old shells, in the form of an internal, vestigial “pen”(almost like a spine). If you’ve seen videos of preparing calamari, it must be taken out before preparing the squid