Other than those things stated it may come down to this:
Ueshiba is said that 'the mind of man is in the hands and the spirit of the universe is in the feet'. Which can be variously interpreted but may be taken to mean that the real movement in aikido is the foot work (i.e. moving & blending), not a hand grappling art.
If you kick anyone you generally put yourself in a position of limiting your body movement (hoping quickly in different directions is difficult to do). Therefore it reduces your ability to blend or react to anything uke does.
I don't know if this is true, but it seems to make sense to me.