Why did my opponent resign?

Because he has developing problems and you can just finish developing and checkmate him.
5...Nce7 looses a piece after 6.d6 Ng6 7.Qe2+
5...Na5 looses because he can't castle in time 6.Nc3 d6 7.Bb5+ Bd7 8.0-0 Be7 9.Re1 a6 10.Bxd7+ Qxd7 11.Qe2 Nf6 12.Bg5 Kf8 (to cover the Bishop via Rd8) 13.Bxf6 Nxf6 14.Ne4 Bxb2 15.Bf6 Nxf6 Qd2 and white is winning.
White can just build up his position and Black can't really defend with all his pieces stuck. Yes it was probably too soon to resign, but he probably wanted to an attacking sicilian and not some sort of passive Franco-Benoni.

I don't think development was the sole reason. I think it was also a bit due to controlling the center

I wouldn't move my knight in this position with black to be honest. Taking the knight looks like a worse move than pushing the pawn to d6 for white. I'd probably push d7 to d6 to prevent it. If white takes it's really not a big problem since white have lost both central pawns and since the b-line is open you can very easily get a rook and your bishop into the game...

So, I think your opponent quit because he plays some kind of setup, and doesn't know how to play without his setup.

Black loses at least a pawn.

You can't post in the forums yet. Play some games!