These are fairly unusual positions, haha. It took me a while to figure it out, but do you mean 2... e6 and 2... c6?
edit: I'm not at the level to answer your question. But it seems you're asking why Slav defense requires different follow up than QGD? Not sure that's easy to answer, but I'm sure some educational content is available on YouTube.
These are fairly unusual positions, haha. It took me a while to figure it out, but do you mean 2... e6 and 2... c6?
edit: I'm not at the level to answer your question. But it seems you're asking why Slav defense requires different follow up than QGD? Not sure that's easy to answer, but I'm sure some educational content is available on YouTube.
Yes -- I meant c6/e6 -- oy! I'm going to go back and edit it. (Thanks for figuring out what I meant!)
And, yeah, you succinctly rephrased my question. Thanks.
Maybe c6 takes away square at g5 and d5 from the knight at c3?
Both 3. Nc3 and 3. Nf3 are fully playable for White in both of those positions (2. ... c6 and 2. ... e6).
The difference in usage is mostly an historical accident... depending on which continuations were found first.
After c6 Nc3 dxc4 e4 b5 a4 black can play b4 with tempo wich is not possible after c6 nf3. I think that is the reason to play nf3 first but both moves are playable
Against e6 starting with Nc3 keeps the Ng1 fexible and the f3 square available. In one of the main lines White's plan is to develop the knight to e2 and play f3+e4. Something like this
Against c6 it depends if White wants to avoid certain variations. For example starting with Nf3 can be useful for White players that want to avoid playing against the semi-slav. It keeps the option for White to play b3 + Bb2 so they put the knight on d2 where the knight doesn't block the diagonal of the bishop.
The ... Bb4 line you mentioned is not a real Nimzo, as Black has already played 1.. d5 instead of 1... Nf6. I guess White has the option to transpose back to a better version of Nimzo lines (for example 4. a3) where Black usually doesn't want to play d5
After c6 Nc3 dxc4 e4 b5 a4 black can play b4 with tempo wich is not possible after c6 nf3. I think that is the reason to play nf3 first but both moves are playable
Clever insight into a sideline concept; yes, this looks like a good reason to play Nf3 first in the Slav Defense. 
I've also heard that Nf3 first versus Nc3 first might be advantageous for White regarding transpositional concepts.
Against e6 starting with Nc3 keeps the Ng1 fexible and the f3 square available. In one of the main lines White's plan is to develop the knight to e2 and play f3+e4. Something like this
Against c6 it depends if White wants to avoid certain variations. For example starting with Nf3 can be useful for White players that want to avoid playing against the semi-slav. It keeps the option for White to play b3 + Bb2 so they put the knight on d2 where the knight doesn't block the diagonal of the bishop.
The ... Bb4 line you mentioned is not a real Nimzo, as Black has already played 1.. d5 instead of 1... Nf6. I guess White has the option to transpose back to a better version of Nimzo lines (for example 4. a3) where Black usually doesn't want to play d5
One of my favorite lines to play in chess
p.s. Botvinnik was a creative player with 11. f3 wasn't he?
After 1. d4 d5 2. c4 I noticed in the games database that:
Why is that?
(In fact, I -- a newbie to 1 d4 -- would have thought the opposite, because 2...e6 3. Nc3 walks into a Nimzo, 3...Bb4, which some prefer not to play against)