((*) -I was going to say 'normal people' but I'm not sure that always applies.
Needless to say every form of cyclist and bicycle is well represented.
However it has been my observation that the #1 thing that makes for a 'bad' (i.e. palpably slow, inefficient, dangerous even) bicycle is the most basic failure to maintain it correctly. Rusty chains, tyres that are nearly flat, brakes that don't work and so forth.
Now bikes of any kind can suffer neglect in this way, for sure. So any bike can be turned into a de-facto BSO.
However there is no doubt in my mind that cheap mountain bikes -the most BSO-like of BSOs, if you will- are by far the most likely to end up in this state. I am fairly convinced that this is at least as much a reflection of the mindset of the owner as it is the inherent quality of the machine. When these bikes were originally purchased, this was often done without significant application of time, money or effort, and when that approach is carried over into ownership, the result is inevitable.
cheers