the 747-SP was built for Pan Am by request, and thats how United ended up with them, although there were other customers. Its main feature was its range, and for the longest time it held that as its strong point, then the 747-400 came along and took it away. They didnt build very many of them. Current aircraft can do what it did in 2 engines, and with greater capacity. The engines CAN be the same RB211's similar to the L-1011, but I believe that was an option. The plane simply isnt as efficient enough as todays planes, even in the hands of a start up. Some airlines still use it for high density longer range routes, but are quickly being replaced by 767's 777's A330, and such.
Hope that helps!