The motive is a mix of:
(1) A lot of kids are already firmly committed well ahead of time and want to just be done with the process. Worth noting this was the 'official' justification that was repeatedly used when the December signing period was first proposed and discussed.
(2) More and more players want to enroll early anyways, since everybody thinks they're going to be a three-and-done to the NFL. But if you're not signing till February, it's a lot more likely you won't be able to enroll till summer which makes it harder to get real playing time as a true freshman.
(3) Coaches self-interestedly want to be done with recruiting this year's class. As plenty of coaches (including Urban) always say, you recruit your committed players just as hard as uncommitted targets. December signing day means you no longer need to focus on those guys and can turn your eyes either to the remaining February commits or to next year's guys.
That said, while it seems like a lot of coaches weren't necessarily expecting it to be so overwhelming (for all intents and purposes, December is Signing Day and February is just a few recruits per school), both players and recruits seem to generally like the December signing period*, so it's not likely it'll go back to just a single February signing day.
*Except of course for the unfortunate kids who commit to a coach who leaves a week later, but the root cause of that is the letter of intent, not the December signing day; even in the old days there were assistants and a few head coaches who'd leave February 7th or whatever.