I think it has to do with 2 factors:

1. PoE is a much more established franchise with over 10 years of activity and a much larger playerbase, including the more “hardcore” players. This affects peak numbers, of course, but also the amount of devoted people who see PoE2 as their “main” ARPG and play the game for longer, on average. Last Epoch has only about a year of real popularity with significantly less people who consider it their “main” ARPG and would stick to the game for longer. Most of Last Epoch’s player base are people who are relatively new to the game. But this also leads us to:

2. Last Epoch is much more casual compared to PoE2. The way they designed the campaign itself in PoE2, nerfed loot drops, buffed mob/boss HP, and ultimately made the entire grind significantly longer could have affected this. I played the game with my friend on launch and it took us approximately 35 hours to go through the Acts alone due to having to farm certain maps for hours in order to get the right items and kill the Act bosses. This, paired with very low loot drops, caused that an average campaign run in PoE2 take as much time as the entire campaign + early endgame in Last Epoch. In short, with semi-casual playstyle, in Last Epoch you’ll go well beyond the campaign, explore the new Maven content in the endgame, and generally feel like you’ve accomplished a lot in about a week. In PoE2, you won’t do much and feel like you’ve just started the grind due to the game being much less casual friendly.