Because you'd need alot of tule reeds to make a teepee and they where usually lashed together in small groups to make mats etc not larger coverings. Make them too big and them become prone to cracking and I think it would be tricky getting them to stay in place.
By using hide(buffalo) it would be easier to secure and transport.
Just a note that Kaya's tribe used the tule mats for their tipis. That's probably why it's odd that the toy one is made of leather - but I can see how the leather is more practical as a toy.