It's because of the complete lack of pay 2 win which a large amount of the player base hate. That and its a bit more challenging and rewarding because monsters hit harder, have more hp, and give more exp
Reboot FORCES you to spend money on the game. You NEED a pet with some skills to grind mesos and you can't revive it with reward points like in regular servers. I've been playing in regular servers for the past two years, i have 3 pets with every skill, a bunch of cosmetics and i have yet to spend a single penny on the game.
If you want end-game dmg in reboot you will hit a wall really fast. There is no bonus potential, no 15 star tyrants, no end-game hearts, no scrolling and almost no good nebs. This is a tremendous loss of range. I highly doubt reboot will be able to defeat Lucid and if they do it will be a 5 mage + kanna party to stack dmg buffs.
Reboot is basically a glorified singleplayer. MMOs revolve around trading and removing that means people interact alot less. In regular servers your reputation means everything. If you scam, hack or bot you suffer from it. In reboot people blatantly hack/bot and no one cares.
If you just want to play the game for a month go ahead and play reboot, its easier early game. But if you want to play for a while regular servers are much better.
Both options kinda suck.. regular servers are super p2w. You don't HAVE to of course, but without swiping a credit card a few times you're pretty likely to never max out or pump out those lines you might see the kids who spent 100+ a week. Meanwhile Reboot it's like... you don't pay for much else besides a pet maybe, but late game you just GRIND. So much grinding for mesos, and even then you might not hit as high as those in the regular servers due to the limits of scrolling/upgrading in reboot.
In the end of both you're left just collecting chairs, fashion, and bickering about which class hits the most lines and damage anyway i'm taking a break from regular servers cause I don't want to spend anymore money, but being in reboot makes me feel like i've been pampered asf in normal servers. Like I can just go spend a few million and get a full CRA outfit. In Reboot just kinda gotta beg and hope someones nice enough to carry you, or keep working for it yourself. I also miss being able to just buy a 2x card .n. makes grinding much slower.
There's so many players on reboot because of all the scrubs that are too lazy to do actual work in regular servers move there thinking they'll be hitting max and solo'ing bosses without effort.
They're tempted by the dream of being pro, but don't really get that it's not as easy as you think, nor is it all that it's cracked up to be.
Basically, lazy pro-wannabes fill half the server. The other half are meso bots.