New versions of Android come out from time to time. The latest is 4.4 (aka KitKat).
This has already been made available for the likes of Google Nexus 7 (which is hardly surprising given that Android is a Google OS).
My understanding is that Google devices such as the Nexus 7 receive a vanilla version of each release and so can get it straight away (it is automatically made available on the device).
Devices from other manufacturers often do not use vanilla versions of the OS (the OS may be enhanced / adapted to work with such devices). So until the vendor adapts the latest release, it won't be available. Some vendors never adapt the latest release(s), so one is stuck with an old version of the OS (a good argument for buying a Google device maybe?).
It may even be that Google will not licence later releases for some vendors / vendors' devices.
I guess you would need to sniff around forums that deal with your device to get the best info on what is / is not possible.