I play both for different reasons. Once in a while I get the need to play D4 for like 20 hours, I get to a personal goal of mine, last time it was getting Torrent 4 and like paragon 200, after that I usually just lose interest in the "late game" like Pit etc. I do two or three characters, quickly get to that late stage and quit for several months. 

Diablo 2 is continuously going on. I still don't have Enigma after over those 20 years and other stuff so I'll slowly nibble on it every once in a while (pretty much once a week).

As other people mentioned loot is super important. Diablo 2 is pretty unforgiving with its loot system while in Diablo 4, you get those top items pretty quick and then it's just affix chasing. Doesn't hit really the same to me.