Disclaimer: I have never personally been to hell.

A number of the fights in hell and other area explores involve unique abilities from the mobs. Countering those unique abilities in many cases requires having or doing very specific things. The overhead of getting that setup for 1 person could be very very different than doing it for a group of 12 (which is about as big as you get today but would be considered medium or small back when hell itself was designed).

There may be creative solutions to figuring out how to tackle these challenges in light of new functionality but it took years to figure out the original game plans and it likely will be years to come up with new ones.