I imagine this is the result of some sort of negotiation with the US government. US government wants backdoors, Apple doesn't want to provide them. Apple also wants to store pictures on iCloud encrypted, without encryption backdoors. The compromise to protect the rest of encrypted things on iCloud was to implement this client side scanning before upload to iCloud to ensure iCloud does not contain any illegal material without exposing backdoors directly to the US government.

In light of that, I then imagine that other providers are already storing photos and data unencrypted and did not object to adding scanning on the server side, and also did not share anything about the scanning with the public.