I think people got too used to bundling by Apple and Google. For most of the core apps there are good and open source alternatives available.

The main point is that AOSP as a system (modulo firmware) is open source and SailfishOS is not. Also, even though Sailfish has an Android compatibility layer (though only for official devices), compatibility is most likely always going to be worse than 'real' Android.

That said, I hope that Jolla Phone becomes a success, more competition is good. Hopefully being funded better will move them to fully open source the base system.

Yes and most people don't realize that the current "AOSP" apps are the LineageOS apps.

A true AOPS image is missing most core Apps.

The Email app had been forked into K-9 Mail, which later became Thunderbird for Android. AOSP Browser no longer made sense to develop after Chromium was ported to Android. And so on. The barebones applications in AOSP have been succeeded by better open source apps outside the AOSP repos. It doesn't make sense to maintain them when nobody putting together an Android distribution would choose to use them over those alternatives.

Because no one was using them. Everyone was replacing them and shipping other apps. AOSP is very modular and customizable letting you configure what apps get included in the OS.