I would guess it's because there was a long time between updates while the material re-design was happening where it stagnated. Then when the re-design finally dropped many features still needed re-implimenting and many bugs needing squashing.

It's been a rough few years for the project because so much needed re-doing but they have come out the other end quite well

Because the redesign in 5.6 made it harder to use the app with multiple accounts and removed features people were actually using for the sake of redesign.

I'm still on 5.403 and I have no intentions on moving.