Not everybody has a short attention span and enjoys change every 365 days (or even every five years). Do not underestimate the value of familiarity with technology, especially for people who consider themselves “normal” (read: not geeks and nerds).
Change can be expensive and/or frustrating. Why fix what isn’t “broken?” Why not make incremental improvements to something that obviously resonates with your users? Too much change at one time is like tossing a frog into boiling water; users often need time to acclimate or you risk (as a product or service provider) alienating them.
Backlash is typically not something anybody wants to deal with. Frustration breeds contempt. So, when I see people kvetching about iOS being “boring,” I want to smack them upside the head and remind them that too much change to iOS would destroy one of the best things Apple has going for it. I don’t care if you don’t like Apple; I don’t care if you don’t like iOS; I don’t care if you think another OS is superior (which is a relative statement). People still do not generally like change if it happens too quickly.