Google went to KC, KS first because they picked them first over KC, MO. Google is first going to the hoods that had the highest number of pre-registrations and this hood (Hanover Heights) had the highest on the KS side.
On the Missouri side, they will start rollout in Spring and most of the highest registrations were indeed high income hoods. KCK doesn't really have particularly high income hoods. Every metro has their lower income areas and KC's is KCK (and KCMO's E Side).
It's prioritized by amount of pre-registrations on each side of state line, not existing ISPs.