What did I do wrong?

Hi everyone. I am a bit of a noob but have been playing some games on Chess.com and working through the lessons. Had a game on here today and after the game I checked the analysis. The analysis engine told me I'd made a mistake but I'm not sure why??

I'm playing White and this was the move it said was a mistake (bc4) :

My thinking was I trap Black's Queen with this move, while also using the Bishop on b6 to protect against the Knight on g4. Sure enough Black's next move:

Then I do:

At which point my opponent does:

And then resigned before I took Black's Queen.

Sorry for being thick, but can someone tell me why my initial move of bc4 was a "mistake" ? And why the engine suggested instead bc7 was a stronger move?

Thanks!

This is why i dont like the online analysis.  Your move is not a "mistake".  Its just not the best move. 

Bc4 was a naturally attractive move because it pinned black's queen to his king. The problem with it is only that there were better moves. After black takes your hanging bishop on b6 and then takes your bishop on e6, you end up a piece up, which isn't as bad.

The first diagram shows where the bishop was, on e2.

The queen is pretty well trapped, anyway, and it won't be easy to prevent or defend against the threat of Bc4, so you don't have to do it right away.  Moving Bc7 first gets your bishop out of the way so that black can't do the in-between move of capturing it on b6 before losing the queen, as well as keeps black's queen out of d6 so it can't avoid the Bc4 attack later.

After your move,

Bc4 Nxb6

Bxe6+ Bxe6,

black gets two pieces for the queen, and those pieces might practically get enough activity to compensate for the loss of the major piece - although the computer doesn't see it with perfect play, a human might muddy things up enough to give white some issues that they might not solve in time.  If you move your b6 bishop out of the way, first, then it will be harder for black to get reasonable compensation for the queen.

So, your "mistake" wasn't that your move was costing you the game, it's just that a different move was notably better.  You're still winning after Bc4.

     You won the queen and your opponent resigned. What more do you want? Many times the computer program says that you have a better move, but it's rarely a mistake to achieve a won position.

     Recently I played a combination that allowed to win the exchange and all three of my opponent's kingside pawns while eliminating all his major pieces and leaving me with K+R+ 5 pawns vs K+B+1 pawn (blocked). The computer said it was better to leave the queens and another pair of rooks on the board and launch a pawn storm, mating him more quickly. 

     Of course that plan would leave my opponent many more chances for counterattack and perpetual check. A bird in hand is worth two in the bush. My opponent also immediately resigned.

This is another reason i wish people wouldnt get so caught up in any engine analysis except for blunders and missed tactics. 

You can have a position where you can mate in 1, or win the queen.  So lest say you miss the mate and take the queen.  The engine will label that move a "blunder".