Probably because the cost of surveillance cameras is more than just the cost of the resolution or storage. the ones that hang outside need to be weather proof and vandalism proof. they were probably bought as part of a large and expensive system, intended to be paid off over 10-15 years[0], to replace all those cameras whenever more resolution becomes available, is probably quite expensive, regardless of how cheap the actual camera is. It's a lot of cameras, in a lot of hard-to-reach places, with expensive reinforced casings, and would the output format of the cameras be compatible with the old system?

OTOH, I do actually see more and more HD (or relatively high def) surveillance footage. It all depends on how when the security system was installed, I guess. Also smaller businesses can afford a shorter turnaround for these things, as well as not having the handicap of a head-start[1].

[0] actually I'm guessing those lifetime spans, it seems a bit much? but I also can't see anyone signing off on a security system that's expected to be replaced in 5 years. good thing my job isn't facility management ... :P

[1] http://en.wikipedia.org/wiki/Law_of_the_handicap_of_a_head_s... , how odd, is there no common English phrase for this law?