Too many network hops! Imagine if an account has an account with 3 folders and want to synchronize all of them. Then need at least to select each folder examine is there something new and fetch needed data. This is at least 6 network calls. You can do that with just one call in JMAP.

Some server support only basic IMAP and do not offer QResync and Condstore extensions. So to detect deleted move email you need to select entire email UIDs to see is it still there. A lo of unneeded network call just to fully synchronize client state.

I even do not want to start that some providers like Outlook don't offer UTF8 extension, so the cannot implement a good search for different languages.