Because 2. C4,... is a stronger move for white. It helps to control and support the center. Then Nc3 is usually played on the third move.
Why 1)d4 d5 2)Nc3 is not a popular option?
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If black plays that same move order from thier side, it is called the Chigorin defense. It is 1) d4,d5 2) c4,Nc6. Now white has three principle moves to continue. 3) Nc3 3) Nf3 and 3) e3
Usually in queen’s pawn games you want to move your c pawn too, but nc3 is not bad.
Same reason the Vienna, Van Geet, Nimzowitsch, Chigorin, and other similar openings are unpopular. The c-pawn is a very useful resource, and while the knight usually wants to occupy c3, oftentimes your opponents’ moves will necessitate Nd2 or c4 Nc3 or c3 Na3 Nc2.
Because of c4, which challenges black’s center while also building white’s own center. If white immediately plays Nc3 it blocks the possibility of c4, but Nc3 can be played after c4. In general Nc3 in this case is much less flexible than c4
Normally if you play 1. d4 you intend to push the c pawn as it can also take control of the center. 2. Nc3 blocks the c pawn though
It's not very popular because you want to move your c-pawn before your knight, but it's still playable and I actually quite like it.
The Jobava London is very popular especially on club level. But the reason why it’s “not so good” is because it blocks in the c-pawn which in turn negates the pressure on d5. You generally want to play c4 and only then play Nc3 for maximum central presence
It seems to be a natural move order or development but there are very few games in database.