Why black is happy with slow development? One important opening principle is to develop your pieces and being ahead in development is the aim of the opening. In the following opening, white is obviously ahead in development (developed the queen and both knights, one of them in a centre square) but black seems to be fine:

1.e4 c6 2.d4 d5 3.Nd2 dxe4 4.Nxe4 Bf5 5.Ng3 Bg6 6.h4 h6 7.Nf3 e6 8.Ne5 Bh7 9.Bd3 Bxd3 10.Qxd3 Nf6
I do not know that I would say "happy", but in chess, there are pluses and minuses to every move we make, and on top of that, there are exceptions to many positional concepts. We see GMs move pieces twice in the opening all the time when there is a good reason.

Now, as to that specific position, black is, indeed, somewhat behind in development, but has compensating factors. In particular, black has traded the bad bishop for white's good bishop, and the exchange of light-squared bishops makes most kingside attacks very difficult to execute, and in the Caro, with less space, that is a concern. The c6 and e6 pawns are well-placed to help control the light squares since the bishop was traded. Black's remaining bishop is probably a little better than white's, and contributes well to the defense.

In addition, black is incredibly solid, so black has the time to finish development safely, and then to break back with c5. It is almost impossible to prevent c5 later.

Lastly, black is normally down one tempo, but here it is two. Black had no choice but to play Bh7 when white played Ne5. How is being behind in development normally punished? By opening the position. That is virtually impossible here, so black is content (the word I would use instead of "happy") to be behind en development temporarily.

Edit: I am going to move a few more things from my comments below to the answer.

One more thing that compensates is that the Ng3 is not well-placed, and often goes to e4 later, giving back a tempo. You also might look at h4 as a potential weakness, but it is rare in practice that it is a factor in that sense...it usually is not weak, but cramps black's kingside.

Development is not the overriding factor in this line, or similar lines...it is that white has more space, which is why black must eventually chip away at the center with c5. If not, it will probably be a slow strangulation.