I suspect it has to do with not having a more current operating system
Exactly, what Apple is doing is leveraging new features only on newer OS X versions to force you to upgrade OS X to 10.7 or later.
What they don't tell you is 10.7 or later will NOT run your PowerPC based software, some of it you can't get Intel versions like for perfectly working printers, scanners and games etc. So it can be a very expensive OS X upgrade to 10.7 or later with third party software.
Also they don't tell you is the risk in bricking your machine if the update goes badly, so you have to have AppleCare in case that occurs so it's on them to fix it. Else your paying money to replace the logicboard or buying a new machine prematurely.
A four year old machine can't have AppleCare, it's only good for 3 years from date of purchase new.
Your machine came with a boot hard drive, it's usually got about 4-5 years of life on it, then it needs to be replaced.
Also they don't tell you the newer OS X version has more abilities and features that causes your machine to slow down in performance, especially on older machines.
Also they implement new, idotic and extremely fustrating new user interface features in a arbitrary "you take it or leave it" manner with little or no fall back method.
So upgrading OS X is a gamble on older machines that otherwise would likely work perfectly fine for the next 5 years if one didn't need to have to mess with it.
Since Snow Leopard has slightly over 25% OS X version market share, your still good for security updates etc for the next few years if you just want to stay where your at and be happy.
The Lions are, well, not so welcome due to Scott Forstall retard UI issues, OS X Mavericks 10.9 is likely going to fix things a bit better.
But basically what is occuring is Apple is trying to force a faster hardware upgrade cycle with the more expensive products by leveraging features on the smaller devices that expire sooner.
Apple is a hardware company that leverages whatever it can to get you to buy more hardware, it's why they are as rich as they are.
... When my wife calls me via Facetime from her Ipad, I cannot click on "accept"- not there? ...
Do you share an Apple ID...
Have a look here...