White was looking for a Blackmar-Diemer, but after 1.d4 c5, the possibility of a Smith-Morra Gambit was there. After 2.e4 cx4 3.c3 dx3 4.Nxc4, White has a Knight developed, diagonals open for both Bishops and open c/d files for the Rooks, all at the cost of one Pawn. Black’s pieces are all on their starting squares and only the Queen has a (short) diagonal.
Black has all his pieces on the Queen-side. White controls the center so it is difficult for Black’s pieces to defend the King-side. White has a space advantage on the King-side and Black’s pawn protection has already been compromised. Thus it is the ideal situation to begin a King-side attack, beginning with 21.Qd3
P.S. The player ratings in the above game are those of chess.com. The game is a turn-based game with 3 days allowed between moves.
P.P.S. One of the problems with playing gambits against the silicon monster is that with a human player, the gambiteer normally gets a benefit from the surprise value of a sacrifice. The computer just evaluates the position and continues on as normal. This makes practice gambit games actually tougher that what is expected OTB.