The queen can be captured by black's rook anytime. Hence white's pawn at b5 needs to be protected somewhat. The only check available through the 5th rank is from white's rook and thereby protecting the b5 pawn as well.

A discovered check by the light-squared bishop (f5) allows greater flexibility than by the pawn at g5. Thus the first move is pawn g5-g6 and not Bd3 or Be4 or Be6.

Be4 g5 to g6 is checkmate too. So why aren't those moves excepted?

As mentioned above...discovered check by bishop rather than by pawn is more flexible and the only plausible move. The bishop can stave off blocks by black's two rooks and the black pawn's move, c6xb5.

If,

1. Be4 Rd1 (blocking move by black's rook)

Or,

1. Bd3 Re8 (block move by black's other rook)

thanks, now I get it.