No rules based reason from my point of view, but i didn't consult the rules at all.
But cleaving with touch attacks is opposed to my picture of cleave. You swing a weapon (best something big like an axe) at an enemy, fell him, and continue your swing thereafter. A touch attack isn't really such a powerful swing, but more an attempt to just touch the enemy. Similar could be said about, let's say, using a rapier or the like.
I admit, that this approach is opposed by the strength modifier given to melee touch attacks, as i would tend towards using the dex mod.
But that's just my imagination of cleave and not based on any rules.
greets, quez