Why was the 747SP not very successful with sales and longevity in service with the airlines that did purchase it?
It just was not a great fit for the use case it was built for at the time it was released:
The Boeing 747SP was hit by the same problem that many airlines have suffered from throughout history… rising fuel costs. The aircraft was heavy, didn’t carry as many passengers as a normal 747 and was expensive to run (four engines after all). Boeing had expected to build up to 200 of these aircraft, but their dreams were never realized.
Additionally, by the early 1980’s, other aircraft started to arrive that could easily perform the same route and provide a better passenger experience. The last commercial flight of the Boeing 747SP flew in 2006 for Iran Air. As the country is currently under embargo and spare parts cannot be flown in from Boeing, it is likely that they retired them out of necessity, rather than just replacing them with something better.
The aircraft, despite its long range, was not popular with airlines, and only 45 were built. This was mainly due to its four-engine configuration, which meant high fuel usage and higher costs for airlines, making it difficult to break even on ultra-long-haul flights. Shorter flights weren’t profitable either, as the 747SP’s high fuel capacity added weight and increased fuel burn on short hops. The aircraft’s failure was not due to a lack of effort on the part of airlines.