Because with stuff like Flash it works out of the box most of the time, while with C++ you get so much inconsistent behaviour even on same platform. For small indie teams or lone developers it's really kinda impossible to have 30 different Windows hardware configurations to test, because your game can go fuck itself on specific graphics card or driver, not even talking about porting.
Well, you're using the same driver with LWJGL or JGL or whatever you use to expose OpenGL to C#, so the drivers can fuck themselves pretty good in any language
But yeah, C++ is more bug prone when you dabble with low level stuff, eg: pointer math can get VERY wrong on different platforms. Some platforms zero their memory, or are less strict about out of bounds access, or won't crash with a double delete, leading to lots of hate when you happen to compile for a platform that's stricter.
Still, using "modern" C++ this is much less likely to happen, and having just ported a very big project I can say that these kinds of problems were the least common.
@_Tommo_: Almost all Android apps are written in Java. I work as a professional Java programmer and we compile to HTML5. There are embedded devices, BluRay players and cars that run Java. Java can be complied to target iOS with some effort. Technically most of these aren't Java because they don't support the entire standard library. But it's really just a matter of using (slightly) different libraries for different platforms, which is exactly what you'd do for C++ anyway.
Cool, I didn't know iOS could run Java, is that a terrible hack or something production ready?
Anyway ok, you convinced me, C++ is the most portable language only by a bit
But let's say that I still prefer C++ for being the "latin" of languages, and for being tied to noone.
Seeing how much each of those Java and C# ports depends on huge efforts from random companies and individuals in emulating the specs of the "real thing", I can't be confortable with it
PS: yeah sorry for the language war
Games can be made in any language, even "indie AAAs" as long as they are 2D or even simple 3D... people even consider using HTML5 for games, so this has definitely to be true