Because clones are never as good as the real thing.
FTG requires chakra.
Clones split chakra, takes more time to use.
Because the edos chakra comes back almost instantly. Unless of course, a highly costing jutsu is being spammed(Madara using two meteors). In a clone, the clone is being summoned with the amount of chakra that was given to them. They're not edo tensei clones. If they were, they'd have to be sealed too.Non edo, this would make a lot of sense.
Clones created contain only a portion of the users original chakra. The more clones you make, the less chakra each has.
FTG, as all jutsus, require chakra to perform. So, if the user has low chakra it would be harder to execute, therefore it would make it slower.
In the case of the edos, where chakra is unlimited, I don't get why the FTG would become slower.
