Sometimes it has to do with momentum; you know, things just keep getting added on. Other times (sometimes the same times) it has to do with budget. Bigger companies seem to have a harder time turning around their networks. One of my clients was a large, large company, and we had to try to convince them to go to 100 Mbit for one group for a few years. At one of my other clients, a much, much smaller shop, I could get them all on Gbit for cheap. (Yeah, I know, not full utilization, but when the tech gets there on the desktop, they'll be ready).
DON'T... call me stupid!