Because it is a paid service, in the world where the web 2.0 services are supported by ads instead, paid services are being out-competed by multiple free services and in this case for mobileme, google's services, MS's Live Services and Zoho and so on.
I disagree, I have an iPhone, macbook pro and an imac. Keeping them all in sync, while possible, is somewhat of a pain. Mobile Me makes it so effortless, when my trial ended and I didnt continue it, it was a massive pain.
True, mobile me isn't for everyone, but I think there are a large number of mac (and pc!) users that have multiple devices that need to be kept in sync.
And for £50 ($99) a year, its actually pretty good value (I think), especially with the added features of mobile me gallery and now chat.
Dan.