In the French line 6. bxc3 Ne7 7. Qg4 O-O 8. Bd3 why 8. ... c4 is not played?

Here are the errors in your "analysis".

1. After 11...Rxf6, Black is not "lost no matter what he does".
2. The move 15...Re8?? is a terrible blunder. Instead, 15...Rf5 gives Black a good position.
3. The move 16. h4?? is a terrible blunder because 16. Bxe7 would've won on spot.
4. The move 16...Bd7?? is a terrible blunder that loses to 17. Bxe7.
5. The move 17. Rh3? makes the win harder.
6. The move 17...Qa5?? is a terrible blunder that loses to 18. Bxe7.
7. The move 18. Ng6?? is a terrible blunder that throws the win away. After 18...Nxg6 19. Qxg6 Nxd4, White must resort to perpetual check via 20. Qf7+ Kh8 21. Qh5+ Kg8 22. Qf7+ Kh8 23. Qh5+ Kg8 24. Qf7+.

Wow. There's some strange analysis going on here.

After 8. ... c4 9. Bxh7? black is at least better and possibly winning. The sacrifice doesn't work because black has time to temporarily prevent Ng5 with f6, as a couple of people have mentioned. The position starts to resemble other main lines with an early f5 by black, except that black is a piece for a pawn up! 

1. e4 e6 2. d4 d5 3. Nc3 Bb4 4. e5 c5 5. a3 Bxc3+ 6. bxc3 Ne7 7. Qg4 O-O 8. Bd3 c4 9. Bxh7+ Kxh7 10. Qh5+ Kg8 11. Nf3 f6 12. exf6 Rxf6 

and now:

13. Bg5 Nd7! 14. Bxf6?! Nxf6 15. Qe5 Qa5 is just winning for black. White's position is incredibly difficult to play. For example, 16. O-O Nc6! (the pawn on c3 can be taken when it's more convenient) 17. Qe3 Ne4 and black can develop calmly with Bd7 and perhaps Rf8. 

I think someone has already mentioned the line 13. Ng5 Nf5 14. Qh7+ Kf8 and white has no particularly impressive follow-up available. He has to be careful of things like Rh6 trapping the queen too. 

Exactly! The move 9. Bh6 is MUCH safer.

May be 8. ... c4 is the typical engine move which no human would ever play. Humans envision all kind of Bxh7 checks, pry open the h-file sac sac mates, closed centers and who knows what. Thoughts in their head just go around and around and they conclude: no way, I won't play this move. Let those play this move who have seven mothers.

And the engine is just rolling on the floor laughing.

Love the idea of chess engines rolling on the floor laughing. Some manga artist should create manga versions of Shredder, Junior, Hiarcs, Fritz, Rybka, Kommodo, and Stockfish. That would be awesome. Then they should draw them all rolling on the floor laughing. :)

I think 8...c4 is by no means a bad move. The move 8...Nbc6 is just more "natural". (Apparently, Stockfish 6 64 likes 8...c4!)

Found these lying around on the web. Shame they're not laughing. :)

8...c4 has been played in master games. It looks like the critical line is 9.Bh6 Ng6 10.Bxg6 fxg6 11.Be3 and black players found this position difficult to defend.

Last game in my database goes back to 2005.

@hicetunc

I was trying to look at publicly available games in the position of the OP (  which is after the 9. Bh6 line is played out to its logical conclusion ), and they were lost by black to terrible blunders. Like black left undefended the e6 pawn and white queen took with check, things like that, on the 2200 level. Not very enlightening games.

@chyss

I'm pretty sure that 8...c4 is completely playable.

Up to move 8 this is the utter main line in the French. All moves are first choices of both humans and engines. If in the 8th move black has a strong move that can't be rejected in correspondence games ( which are engine assisted ) this says something about 1. e4. If we can belive Fischer that 1. e4 is best by test, then this says something about chess. Then this is a piece of evidence that white's advantage is very short lived and can be neutralized in very concrete terms.

I have a new line against 8...c4 utilizing the Bxh7 motif. It seems you CAN play Bxh7, but you have to be able to play h4 in the case of f6, where black's pieces do not seem fast enough to stop h6 and subsequent mating patterns. This is not conclusive, but merely an idea. Tell me(in kind words, thank you) what you think.

@Aquarius550

10. ... f6 was the correct defensive move after 10. Qh5+. If black has the luxury of not having to meet this check, then 10. ... Rh8 can be played, which stops ideas on the h-file in a very direct way. In fact white cannot avoid this check, so in your analysis you should start from 10. Qh5+.

That's right. What's the point of 9. Bxh7+? If White's knight was already on f3, the sacrifice would work. However, in this case, it isn't. It's best to stick to the simple 9. Bh6.

Pfren was just faster. Sorry about that.

When you sacrifice two pieces for two pawns, you MUST have some compensation. White barely has any compensation for the lost material. With proper defense, Black should easily win.

@Aquarius - good job doing these analysis by yourself - it has become rare as everybody is firing their engine these days. I command you, and I think it will benefit your chess in the long term.

For the Bxh7+ sacrifice to work, you need a knight that can go to g5. In this case, White's attack is one move slow. Therefore, 9. Bxh7+?! gives Black the advantage. I'm not sure how the sacrifice favors White. White has some compensation, but not enough compensation to consider it completely sound.

I looked at Rh8, but I'm still working on it. I actually, believe or not, think white is winning with the two pieces off the board. If black could organize his pieces it would be different. Buts its almost like white has all the time in the world...

I've got an improvement one sec.