I can think of three main reasons:
1)Pan Am financial difficulty. They needed to keep costs low and revenue high to stay afloat. They didn't, and I don't know if the 762 could've helped. The 763s are pretty efficient but the 762s were much less so, especially the early versions.
2) The A300. They already had the A300, and it fit them perfectly. No real reason to change, and it could be expensive.
3) The 762ER wasn't ready immediately. The 762 could fill most then present gaps, but wasn't better than the A30) for them. Extra crew, mechanic, and ETOPS Tim's and money. Look at WN
or most LCCs today.
You answered your own question somewhat. For them the 767 couldn't do really anything the A300/310 couldn't do, and they already had the airbuses. Had they made it through 1991 and 2001 and 2008, I wouldn't be surprised to see a long haul fleet of A330s, with A320 short haul and quite possibly the 747 or 777 for high capacity Long Haul. But I could see them using the A330s like some 767s are today. Also, they would probably have new passenger A300s, or at least circa 2006. They still wouldn't have a reason to get the 767.