Mostly just his attack range and his headshot. In the late game, his attack range really helps with several waves, such as against the birds, and most bosses, as he can initiate them outside of their attack range. Then also in the late game, he will have so much attack speed, he will be able to headshot within the time that headshot will have worn off, so essentially the enemy he targets will be permanently slowed by 100% and lose 100 attack speed. So on much later waves where it is just a few very strong bosses, he can permanently slow one in addition to dealing fairly high damage, such that he may be able to kill one of the bosses before they even reach the first tower. Plus as a ranged agility carry, he benefits greatly from the new manta style, as it grants him something like 70 extra agility, so his firing squad of illusions hit very hard.
Also there is the fact that he possesses an AoE damaging spell that is pretty strong in the early game, so he doesn't fall into the trap that many agility carries have of sucking in the early game.
Then there is also the fact alone that he is a ranged agility carry, which there are only about a dozen of, and even less than that are very useful in this mode.
Drow is decent late game but lacks an early game presence, and needs another ranged hero to really be useful. Luna can barely do anything early on due to her pitiful attack range and the glaives not dealing much damage, but is very good later on. Viper can't do that much in this mode as his damage over time really trails off and his passive damage increase doesn't scale very well. Medusa needs a lot in order to be able to do very well. Clinkz can't do much except for single target damage, and is too squishy to survive, and most of the later waves have no creeps to eat so his main source of health and damage is lost. Not really counting troll and druid because they are better as melee heroes. Venomancer's main source of damage is his damage over time and wards, both of which fall off quickly, though he has some utility in that the wards can be used to distract the enemy.
So for ranged agility carries that do decent early and late game, you have the choice of sniper, mirana, morphling, vengeful spirit, templar assassin, gyrocopter, shadow fiend, razor, and weaver. Of those heroes, sniper has the simplest skill set and is the most easy to use. He doesn't require careful positioning like TA, shadow fiend, and razor do in order to deal the most damage. He doesn't have a lot of skills that need to be constantly used. He is simple and effective.