So Intel now has S775, S1156 and S1366, all of them incompatible with each other, yet nobody complains about that?.

AMD had 939 and AM2 at the same time and it was an unforgivable sin. Then came AM3 (AM2+ is just chipset related, doesn't really count), which has backwards compatibility at least from one side to the other, and once again some found it to be an unnecessary pain.

*Why* is that?

Maybe since Intel has done this since the begging is nothing unusual or unacceptable for them ...