I would guess it is due to a couple of factors:

RAM can now be tested in one package on the motherboard and people can't accidentally change it. In the long run it may open some abilities w.r.t using RAM for other things since you know its exact clocking.

The production is cheaper since the MB does not need an extra "install ram" step which is probably a human.

You can probably win some height space by a couple of millimeters.

You have more control over heating.