For the majority of users, they don't care. But you are on a tech forum here, where people want the latest and greatest.
Why do people upgrade their Windows or OS X machines? Because there are newer versions, with better features.
Why don't people upgrade their Windows or OS X machines? Because they aren't aware of upgrade or the current version does what they want need - don't mess with a running system.
Why do people hang fuzzy dice on their rearview mirrors? No, honestely, why? I can't think of an answer.
I got my update yesterday and my Sensation is running faster and smoother than before and there are certainly some usability improvements which made upgrading worthwhile.
For a lot of users, like my brother-in-law, the updates are irrelevant, he doesn't even use the web browser, address book, calendar etc. He only has a Sensation because his provider foisted it onto him, he'd be a lot better off and happier with another Nokia dumb phone.
I really think that the manufacturers need to get their act together and at least get security updates out within a month of them being released by Google. Anything else should be seen as verging on criminal negligence.