It has to do with the reset and early LP inflation from the start of the season
Two biggest factors (from one of the designers):
(1) Due to the early season LP issues we saw, where high skill players were earning LP fairly quickly, Diamond+ got fairly inflated. When we had the reset earlier this season, we floored the reset at Diamond 4, meaning no one was put lower than that division. This pushed most of that inflation back towards D4.
(2) There's also a general tendency of people to hit a benchmark and take a break from Ranked - division 4 tends to be larger regardless of tier, but this becomes even more true as you move up the ladder.