They probably also thought (understandably?) that the use-case for a tablet is more likely to involve sharing than a phone.

The UI is on the lock screen, so plenty of room. More likely (1) Honeycomb style rush job and (2) "everyone should have their oen phone"

Well, it makes a little less sense on phones, since unless you switch the SIM card, you are still treated as the other person as far as the phone network is concerned. It would be a little odd to be logged in as one person, but the phone network treat you as the other. And since there are features like importing and exporting contacts from the SIM card, those would be more complicated in multi-user scenarios.

I think there's a certain sense that a phone is a much more "personal" device than a tablet. While it would probably be possible to add multi-user support, it would be more difficult, and be less useful, than on a tablet.

While I personally find it annoying, I know many parents that hand their phone to their young children to play games or music. Multiuser support is very much in demand on phones, even if it offered nothing more than a safe guest account.

With kid's corner feature in windows phone 8, you can have fine grain control over what the kids have access to.