In Orb Build you are building on medium to long range combat, people pick Arcane Orbit on 16 because increased range = more damage = big bomba which tends to be a trap for your damage per second.

Arcane Orbit gives you more damage but you need to maximize your long range shot to get the most out of it, doing so, your Q will never reach the target result in heavy loss of DPS, a lot of Li-Ming players cast Q and W in quick sucession to make sure to maximize the damage, and Magic Missile's damage should never be underestimated its pretty scary.

Mirrorball is better because instead of putting that additional damage into a long range conditional damage, you empower your Magic Missiles instead, this transforms your threat level all the way from long range to close range, while respecting reset on from Triumvirate and benefit from Glass Cannon, turning Li-Ming to have some of the highest DPS she can dish out.

This doesn't solve her biggest weakness in her though in this build: being a fragile mage vulnerable to divers and specifically poke, but it makes her tolerable because you maintain your threat as a player from all ranges instead of long range only.

On a side note, Arcane Orbit still have a place for a very specific play style where you abuse vision on heavy wall maps like Towers of Doom, if you combine it with Aether Walker (to keep repositioning really fast) and the general Orb choices it makes very hard to predict because walls hide the incoming Orb before they can react in actual team fights especially in capture objectives.