NiMh batteries

Any reason VEX doesn’t use Lithium Ion instead of NiMh?

because all their batteries are NiMh

NiMh is cheaper.

And when the Cortex was designed, NiMh was MUCH cheaper.

NiMH are also safer. They have less active materials compared to Li-ion batteries. NiMH can pop if they are overcharged too much or short circuit but this is nothing compared to a Li-ion which can potentially blow up.