Chess principles. Also, everyone tells them to decline it. You need to be careful if you play the QGA, while in the QGD, it's a quiet, boring game with no reason to worry or rush.
If you decline the gambit with e6, you commit to a solid center, prepare development and prepare to castle your king to safety after Nf6 and Be7. If you accept, you give the center and trade a center pawn for a flank pawn- which is a little bit unprincipled in an opening. You could argue that white trades a center pawn for a flank pawn in the Open Sicilian, a highly respected opening, but we don't talk about that.
Yes but black trading the c pawn for the d pawn in the Sicilian is almost impossible to avoid for white. Obviously there's c3 lines white can play, but the c3 Sicilian is not amazing for very concrete reasons
QGD and Slav are very hard to play as Black. So I swithched to QGA. I won a game in an otb game against a seasoned active tournament player. He knew how to play but I have more experience in QGA online games so i know the plans more. I am fide unrated. I lost 3 of my white pieces games lots of Caro Kann players! Waaaa.