I think the reason why they didn't do it was becuase it was just a dumb idea, Kermit is really Tex Richman in a Tex R. costume? Nope. Doesn't really fly for me.
I think part of the reason would also be becuase it would ruin kids thoughts on Kermit. If you said Kermit's not allowed to lie it'll make much more sense. But then we could probably think of times where Kermit lied too.