The Positive Test Rate (PTR) appears to be a key discriminator. The limit they set is over 10%, which is exceeded by the banned states. I don't know what California's is.It’s not actually a travel ban, it’s a 14 day quarantine for people arriving from those states. Texas and Florida both had something similar but it is strange that California didn’t make the list.
When the PTR is high, then you know you aren't testing enough and that there are many, many more infected than what your test results show.