The answer may be related to Pegasus software.
Timeline:
1. Leaked documents show Pegasus software exploits all iphones using an exploit in iMessage
2. Apple releases security update (doesn’t patch imessage. exploit)
3. Apple announces CSAM client scanning coming soon
4. Apple releases another security update (still leaves iMessage exploit unpatched and used by Pegasus)
….
Perhaps Apple is under pressure to provide a back door prior to patching a tool that may be widely used by governments around the world.