On Lichess, everything is free, and it has the majority of the features present on Chess.com, even shining brighter in some facets. For example, on Lichess, you can export all of your game with the click of a button (see your profile) into a pgn file, so you're never stuck to the site. There's also the analytics page on your profile, that shows you incredibly detailed, filter-able statistics on your play and playstyle. There's also all the free studies that can teach you countless openings, and concepts too, like the Lucena/Philidor positions. I've played, and spoken to hundreds of people (I used to run a chess club) about the differences between the sites. People seem to believe that cheaters are less prominent on Lichess than on Chess.com. Also, you don't have to pay for analysis, BUT you don't get access to briliant moves. Honestly, if Lichess added brilliant moves, I think tens of thousands would finally flock over. But brilliant moves aren't a very rigid concept, as "better than engine" moves are basically impossible, and it's just an algorithm set to determine whether a move was interesting in the eyes of the developers or not. And it's probably patented. Who knows.

People also complain about Lichess's ui for some reason. They say it's too archaic. Anyway there's a plugin for Firefox and Chrome, of course, to fix that. I used it for a while and it's nice.

There's also the whole world of rating. I've heard so many people complaining that Lichess ratings are "inflated", and they use that very word. They cite Chess.com ratings as being more accurate, but accurate to what? Accurate to FIDE/USCF is what they mean, and the basis of that argument is upon system-to-system comparisons of rating, or in other words, comparing ratings between one website/federation and another website/federation, which does not work. Rating systems are finicky things, and the rating curve of a system is dependent completely on its players. Just setting an average rating is not enough. Every player counts. People also think of Chess.com's rating system as better objectively than Lichess's, but the opposite is actually true. They both use a variation of Mark Glickman's Glicko rating system, but Chess.com uses the archaic Glicko-1, whereas Lichess uses the newer Glicko-2. Glicko-1 is archaic because it doesn't include the volatility field in a player's rating, which represents the chance that the player creates an upset (losing to a much lower rated player, or winning against a much higher rated player), and it's important in calculations. For those who still are insistent on system-to-system rating comparisons, there is a Firefox and Chrome extension that puts Chess.com rating equivalents side-by-side with the Lichess ratings on the Lichess website live. (For more information on Glicko-2, see Mark Glickman's Paper).

The main reason, other than brilliant moves, for people using Chess.com, in the hundreds I've spoken to, is the fact that they had heard of Chess.com first, started paying for it, and now don't want to move because they've already committed themselves. That's basically it.