There's not a strong reason to use Mojo over cython right now, but if Mojo can deliver on their claims, I think there will be. A borrow checker, better IDE support, function overloading support, and better SIMD support are things that stick out to me in Mojos favor.

Aside from the rest of stuff they offer (and higher speed, if they're telling the truth) Mojo is also a standalone language you can use, whereas Cython is meant for dual-mode use alongside Python.