I seem to recall from a long time ago a discussion about when the ground can cause either a fumble or an incomplete.
Basically if you're effectively stable (you can walk/run/stand) and you catch the ball, merely hanging on to it long enough to prove control is enough to call it "complete". But if you're diving, you must maintain control of the ball through contact with the ground because "control" isn't assumed until something else happens (the next step, getting both feet down inbounds, getting hit, etc).
In the endzone, the question becomes whether control plus dropping the ball either equals touchdown or incomplete. They decided that once again, you must prove you actually have control of the ball in the endzone for it to be a touchdown, so you must keep the ball through the next event. If you're diving, the proof that you have control is if you can hold on to the ball through contact with the ground, no different than having to maintain control while getting both feet on the ground inbound for a catch near the sidelines.
It makes sense to me, and it closes what would otherwise be a blank area in the rules.