Because it was taking too long. As celebrations got more and more elaborate and more and more frequent (for a while there every “big” play would get a celebration) they were adding pointless time to the game, interfering with the broadcast, and just generally making the whole thing boring and silly. So they clamped down, first by telling the refs to get the ball to the spot quickly thus ending the play clock “pause” that allowed players to get silly. Then came the unsportsmanlike conduct rule for TDs.
Save the celebration until the clock strikes zero, until then be like Jerry Rice, hand the ball to the ref and get ready for the next play.