The phone manufacturers / carriers are trying to use newer OS releases as bait for you to purchase a new phone. Their treating the OS as a "feature" and not as a user upgradable item. Some manufacturers (Motorola) go through such great lengths to actually lock the user out of being able to update / modify the OS on the phone. Next phone I'm going to buy had better be user up-datable to whatever the fck I want.
This is one reason why i do not like the Android platform. They are CONSTANTLY adding new features which require more beefy hardware. It would make much more sense to work with system specs in mind that cater to the majority of your user base, instead of only pleasing 30% and pushing the other 70% to upgrade their phone.
If you're desperate to update your phone there are quite a few sites out there which specialise in upgrading the ROM's on android phones just google your phones name and add custom rom to the end and you'll find about 15 or 20 sites which do it... Takes a while to figure out what to do, though if you read the wiki's you'll be up and running in less than half an hour. I'd recommend xda developers. Have fun or not - the choice is up to you!
Just to add to the above phone manufacturers often add a whole load of skinning software and other tools and restrictive software to their phones, which often hinders more than helping getting a more vanilla version of the operating system is probably better...