One possibility is that they'd like to go E2EE with all iCloud data, including photos, and this lets them do that without the politics (DC level) falling out against them.
I doubt it, considering they give up customers' data when the government requests it for 150,000 users/accounts a year[1]. Not only do they choose to give data when requested, they have to comply with court orders, as well.
Also, governments care about much more than just CSAM. They care about terrorism, drug manufacturing, drug and human trafficking, organized crime, gangs, fraud etc.
This speculation only makes sense if Apple intends to use their CSAM detection and reporting system to detect and report on those other things, as well.
Governments won't like that either, because during investigations and discovery, they'll want all of the customer's data that could be evidence of crimes. These detection and reporting systems, as implemented and by themselves, are only useful for flagging suspicious people. They're no good for complying with warrants or subpeonas.
I’ll take the other side of this and bet that Apple offers the ability to remove themselves from escrowing iCloud keys before they expand CSAM detection to other kinds of content. Nerd points are at stake.
