Why is it not USSD?

USSD is great, but for some reason has not been picked up to any large extent by the mobile operators.
They stumbled across SMS, but failed to extend the lesson to other areas.
Sure, some operators use USSD. There are roaming services that allow you to call by prepending a USSD code to the number you want to call.

Operators in Thailand have a great USSD recharge method for prepaid mobiles (dial *120*number on recharge card#), but in general it's not used as much as it could be.

In the US, it probably has most potential, because people are used to the number->letter transformation. Imagine *120*STARBUCKS# to give the address of the nearest coffee shop, or *175*METS# to get the latest score.

Or how about some Presence information as a USSD message? By setting up some USSD codes I could switch my presence status on a telco server.

USSD has a lot of limitations, but so did SMS and it hasn't done too badly!