simply because the queen doesn't belong in the centre of the board at the beginning of the game where black gets an easy lead in development due to the queen being subject to attack. if you want to play 2. d4, after exd4- c3 is probably a better option, leading to a position similar to the smith morra gambit against the sicilian. in any opening there are 3 things you are trying to do as quickly as possible- control the center, develop your pieces (signified by the rooks being connected- meaning directly protecting eachother), and securing your king's safety (usually by castling).
in the line you've given, white has moved his queen twice where that time could have been used to develop the minor pieces. obviously it is playable for white, otherwise no-one would play it- but it is certainly sub-optimal.
Pardon me if this too basic, but I was going over one of my recent offhand "blitz" games using a master's database and found that the standard Centre game (1. e4 d5 2. d4 exd4 3. Qxd4 Nc6 4. Qa4?! Nf6) ended up faring poorly for White. I thought this to be counter-intuitive. But the master's database on this site at least has black winning 61% of the time after 4. ...Nf6!
Seeing as how most of the Centre game openings in this line end at the position notated above, I would ask why does White ever, ever, play this opening. It is, however, quite common in beginner games that I play.
Enclosed is a .pgn diagram:
And sorry if this has been covered before or is common knowledge. I'm kind of a beginner and certainly no opening theorectician. Thanks in advance.