Why 1.d4 is very popular move now?
The question is how to play d4 opening? Here is the example. This is one of immortal games of GM Alexander Sysoev from Gubaha (Perm region). 1...d5 is not common move now. The most popular move is Nf6. Why Black plays Nf6? Because it's more flexible move, more flexible opening, one of such openings: King's Indian Defence, Grunfeld, Benoni, Queen's Indian Defence and Queen's Pawn Game in brief (2...e6). There're many systems and variations. But 1...d5 is invitation to Queen's Gambit. And the second most popular move is c4, but White can play also Nf3 or Bf4. All ways of Queen's Pawn Game are calculated. Why d4 is very popular as first move? Because it gives White the balance and comfortable playing in the Middlegame and Endgame. This opening is very old and it's approved as one of the best openings. d4 is powerful, strong beginning. If player chooses this opening, he knows what to do. But I know many Grandmasters, who avoid playing d4, because of Nf6 and Indian Game. Nf6 is very effective weapon against d4 at all levels - from beginner to GM. Grandmasters, who avoid d4, play e4 or maybe Nf3, or c4. 1.b3 is good move too, but it's not very popular. Play d4, if you don't want to play against Sicilian or French, or Caro-Kann Defence. Play English (c4) as alternative opening instead d4, but c4 is connected with d4 and these moves together are strong and important for developement. Play Reti (Nf3), if you don't know, what opening to choose. Your opponent will choose it for you. But nowdays d4 is in favour. Play it, and you will be able to beat your opponents in Closed Game. d4 is always Closed Opening. Closed Game is more calm with careful, tactical moves, than Open Game with many sharp moves and variations.