It’s a great question – my suspicion is that a lot of businesses don’t want to go through the annoyance of trying to run something even vaguely custom. They’d much rather buy something “off the shelf” (so to speak) with lots of promises from salespeople.
The allied question is why upgrade when you have a solution that works? We hear stories about organizations still running off floppy disks etc, but if it works why take the risk and go through the effort of upgrading it? If it becomes unrepairable or the supplies aren’t available, then sure. But otherwise? Especially with something that’s not connected to the Internet and thus without serious security risks…
But really, I’m just posting to get to the point where I can add one of my favorite news stories of the last two decades – the business in Texas still doing (as of 2010!) their payroll on an IBM 402, a machine released in 1948!