Xerath is popular because he's basically a siege cannon. He pumps out a lot of long ranged damage, sure it's all skillshot based, but if you hit 3/4 of your skillshots (Since it's almost all AoE damage) you rip an entire team to shreds.
Xerath is also a safe pick in most cases, the only champions that beat him, are really the champions that beat just about every mid laner. He has innate mana sustain through his passive, can harass and last hit entire waves simultaneously if ahead. When your team is behind, he can hold waves off of your tower until the end of days, and when ahead, he can keep them shoved behind the tower and the champions running around like they're dodging sniper fire (because they basically are).
My biggest reason for enjoying him so much, is that he's exceptionally satisfying when you do well, because EVERY SINGLE ability is a skillshot, so if your skillshots are on point, you do well at long, easily-zoning-the-enemy-from-xp range.