Because I was originally writing some very CPU intensive SIMD stuff, which Mojo is also fantastic for. Once I got that working and running nicely I decided to try getting the same algo running on GPU since, at the time, they had just open sourced the GPU parts of the stdlib. It was really easy to get going with.
I have not used Triton/Cute/Cutlass though, so I can't compare against anything other than Cuda really.