One could also ask, why don't free software and Ada mix? We have a first class Ada compiler, it's a clean, readable language, far more elegant than C++ IMO. But, like C++, it has a (largely undeserved) reputation for bloat.
The GNU Ada compiler is not yet first class, it is not in CVS GCC and most distro don't have it. This should change real soon now. And I guess there are a whole bunch of existing and new Ada free software that will gain visibility as a result, not the least being the GNU Visual Debugger. Also Ada Core Technology is one of the few 100% free software company making money.
Once a programmer has been crowned "hacker" by his peers (which is quite often what he was aspiring at in the first place) that makes it even harder for him to ever reconsider his technical choices (especially if that involves significant effort like learning C++ is). So if he's ever had bad experiences with C++, he'll stay firmly on his positions that the language just isn't good.
There's also the fact that many of the "masters" (Miguel, Linus, RMS) have publicly stated that they don't like the language. What's more, no hacker as high-profile as Linus' or RMS' is using C++. And as egnor says, almost all successful Free Software projects use C, and thus somehow prove that C is "good enough".
May be those "masters" have good reason not to like the C++ language, I mean other than just being lazy and unwilling to learn, don't you think?
I've attended two RMS talks, at the first one at a direct question about what he thought about C++ he called the language an "abomination", at the second one he made a joke about C++ being ugly that drew applause from the crowd.
About RMS ability to understand quickly a "complex" language, I would remind that he just looked at the Ada Reference Manual and invented a new compilation model for it that is now used by GNAT and commercial compilers (he just didn't like the traditional model ;-).
I think that saying that "high-profile hackers" don't like C++ because they did not learn or understand it or are unwilling to evolve their technical opinions is questionable.
As for C++ at the workplace, I work for a bank, and as some of my coworkers were quick to adopt C++ a few years ago, it looks like they're dropping it even faster in favor of Java (they say C++ is legacy...).
I certainly think people working on free software (most of them on their free time) are choosing their programming language following their own taste or curiosity, and so the mass dynamics found in the workplace doesn't apply and freedom of choice works 100%.
This together with the slow progress of g++ looks like a good explanation of the dynamics observed by egnor.