Why did the Mets' Shortstop drop the ball on the ground instead of catching it?

The infield fly rule says that any fly ball hit in the infield with 1 or less out is an automatic dead ball out. This is because back in the old days, unscrupulous players would intentionally drop a fly ball like that, and just turn a double play (which is what Reyes was trying to do). By invoking the infield fly rule, the umpires astutely prevented Reyes from cheating.

Actually it was a pretty funny play. He hoped to turn a double play but the ump was right on top of it and saw that he had indeed caught the ball for a line out. It wasn't a play the infield fly rule applies as there was only one man on base. For the infield fly rule to be used, at least 2 runners need to be on base. The assumption with only one runner is that the batter would advance, so there would be no advantage in dropping the ball. The play tonight was similar to a ground out (at least that's what the shortstop hoped!) and it would be a 6-4-3 double play. Instead simply a line out. I did enjoy the play and the heads up of both the shortstop and the umpire. Baseball, what a wondrous game.

Because he is a cheater and tried to get away with it.

you should walk up to jose reyes and ask him yourself