When the 747SP was designed in the early 70s, twinjets were not allowed to do long overwater flights, and as a result there were no widebody twins. Pan Am wanted an ultra long range airplane for its New York to Tehran route, which at the time was the longest in the world. The 747SP was shortened to remove weight and increase performance. An extended range trijet, like a modified L-1011 TriStar, might have been able to do the job with better fuel efficiency, but fuel was cheap then, and the 747SP allowed maintenance commonality with the existing 747 fleet and could still be flown on a 747 type rating.

If n the mid 80s new ETOPS rules were introduced that made it possible for suitably certified twinjets to make ocean crossings. At that point the 747SP became obsolete, and all three and four engined airliners were on the way out. Airbus developed the A300 and Boeing the 767 as the first twinjet widebodies. Had these existed and been legal to use in the early 70s, Pan Am would surely have used them. But they didn't and weren't, so we got the 747SP.