E2EE would have made it significantly harder for Apple to build the web based apps at iCloud.com. Not to say that shouldn’t have though, but I can understand whey they didn’t.
I think the main thing is Find My access, but Apple seems to claim it's E2EE despite being available at icloud.com/find so perhaps it wasn't too complicated; I imagine it stores the plaintext password in memory to access the data.
But I imagine the FBU wouldn't like an end-to-end encrypted iCloud Photos at all.