Are you implying that correlation implies causation ? Maybe people don't stick with older versions of OS X precisely because Apple drops support ?
Maybe enterprise customers don't want to migrate off because of application/hardware support, a hurdle Apple doesn't quite face in light of poor enterprise support ?
First of all, unless your hardware does not support the new OS, there's no reason not to migrate with Apple. I don't remember any OS X releases which actually ran worse than the old one on supported hardware. On the contrary, until Leopard, each new OS X ran faster on the same hardware compared to the older one. So there was a huge motivation to migrate immediately simply because one was getting a faster computer for 129$.
Second, many popular apps drop support for old OS X's very fast on the mac side, so people have to migrate if they are using those apps. There isn't many professional apps out there which run on Tiger anymore unless you want to stick to the old versions. And that has nothing to do with lack of security updates. Look at Photoshop, one of the most popular mac apps ever, it doesn't even support Leopard. 10.6.8 or later only. But the same version supports XP on the win side. Maya is 10.7 or later only already and supports XP on the win side, again.
So if you are a user of many popular professional apps, you have to migrate at least to the second latest OS X for that reason alone or keep using the old versions of your apps.