From my perspective, the reason is servers. Many servers can't update because the versions 1.13-1.15 are completely unplayable for large servers. I play on 2b2t, and its more likely that it will update from 1.12 in the next 4 years than it would have been for the last 4 years, and there has been plenty of testing, but I really won't hold my breath for any final results.