Same logic as to why they are nerfing black king bar in dota for several years. You cant have a one-solve-everything item, that is also gives you good stats and easy to buy. They both occupy active slots so its not like you will get both of them and they are for different situations.
If unstoppable had effects of remover it would be too good, if remover was lingering effect like unstoppable it would be too good, if it was a one item even for a premium cost like boundless spirit, it would be must buy for a lot of heroes.
Unstoppable is when you need to carry out certain actions but they will be interrupted (you are ulting as haze, wraith or seven)
Remover is for when you hit and run, so you wont get stuck with root or slow on you and keep running
Unstoppable still too good in comparison to remover as it renders mo&krill or wraith or others mostly unable to do anything
But remover needs to be in the game, i would only think of improvement if it could be made into ally targetable like healing rite to give it some identity.