USSD – The missed technology ?

USSD (Unstructured Supplementary Services Data) is a technology build into the GSM standard as was SMS and is, indeed, very similar to it. The main difference is that while SMS is store&forward, USSD is session based. This makes it very similar to WAP.
My main question is therefore, WHY this technology was never adopted (since it’s available for years) and those it still have potential?