Ableton not supporting VST3 for a long time was probably a big reason in the delay too. It was only last year that Ableton got support for it.
Also, i’m sure that plugins that relied on a MIDI CC style ‘hack’ with VST2.4 could no longer perform the same functions in VST3 as it was stripped out.
However, we’re now approaching the age of MIDI 2.0, so VST3 should very much take the reigns from now as it’s agnostic vs VST2 which was routed more in MIDI.