Abyssal Blade Tinker ?

 

Why is Abyssal Blade not picked up on late-game Tinker? With level three Rearm the Tinker can keep anyone stunned until he runs out of mana. This even goes through Black King Bar. Why do people not replace their Scythe of Vyse with this item late game, since it offers better lockdown (no movement, unlike hex) and goes through Magic Immunity?

The main reason is that the disable on Abyssal Blade is melee range

Which is much less convenient for a fragile hero who really wants to Boots of Travel to a nearby creep and instantly disable someone when he arrives. You could Blink onto them, but you're putting yourself in a situation you don't really want to be in. In teamfights especially you want to be mobile, not stationary next to the hero you're disabling.

Add to that the much shorter duration - 2 seconds instead of 3.5. That means you're re-arming twice as often to maintain your stun, so you're chewing through mana twice as fast.

To top it off, Abyssal Blade offers very little of use to Tinker other than the stun

He's not really a right clicker, so the extra damage isn't really relevant and the bash chance on ranged heroes is too low to be reliable. Hex meanwhile gives you mana pool and regen, meaning you can do more between trips back to the fountain.

I can see situations where you might do it, for example if you're focusing entirely on picking off solo targets where you can Blink right onto them without a problem. However in that situation you probably have the element of surprise too, so magic immunity probably wouldn't be a problem in the first place.