Some people may not be able to afford it, or don't have a Mac capable of running it?!
To be fair, the question is why people are running Leopard on their dual-core Intel Macs. This naturally excludes all PPC Macs, and all the upgrades after Leopard are $29 so far. Thus not being able to afford it is unlikely as well.
If the question was why anyone at all is still running Leopard, this would be a valid response.