Also, many of them feel that updating drivers is a difficult thing to do (too technical for them) and are afraid that they will mess up their system.
Honestly, this has mainly to do with one of the most common recommendations to install drivers, which is to go to safe mode, uninstall the old driver and install the new one. You don't have to, but it isn't hard to see where the issue lies. Besides, people think that if it already works as-is it's probably fine, not realizing that old drivers may be leaving features unused (e.g. the drivers bundled with the GeForce 7 use OpenGL 2.0, but the newest drivers provide OpenGL 2.1).
In reality, there are a lot more games that use DirectX than use OpenGL (easily 10 to 1 ratio). So, Intel/AMD/NVidia have not had too much incentive to keep the quality of their OpenGL drivers on par with the quality of their DirectX driver. But, the quality of the OpenGL drivers in the past few years has greatly improved.
It's a chicken-and-egg situation, if nobody uses OpenGL, there's no incentive to improve its support, which in turn means nobody wants to use it, and... well, it's a self-feedback loop. I think id is pretty much the only reason it didn't die completely. At least OpenGL 3 seemed to have gotten all vendors back into OpenGL, just because apparently it had enough of a reputation to make lack of support look stupid (maybe the backslash when Vista was implied to lack OpenGL support was a hint, even if it turned out to be false later).
The bad news is that a lot of people are still using (or stuck with) older, buggy OpenGL drivers.
I wouldn't expect those to care about gaming anyway ^^; (or to have something that supports anything newer than 1.1 for that very reason...)
EDIT: basically, if you care about people with old systems (especially people in e.g. developing countries, where hardware can be considered quite expensive), OpenGL 2 may be a good compromise. If you expect some decent hardware, OpenGL 3 would be better. I'd say that OpenGL 4 would be better if considered optional for now unless you really need the most powerful hardware (i.e. support it if you want but don't assume it'll be very common).
If somebody is stuck with OpenGL 1 that's most likely the kind of people you wouldn't want to bother targetting anyway... Either their hardware is pretty weak and will slow down without much effort or they're the kind of people who'd rather stick to browser games (if they play games at all).