We already discussed it before. Most common believe is very long US warranty for batteries: running pure EV mode too much would keep the battery at very low state of discharge and shorten the life of batteries, presumably increasing their failure rate before warranty expires (up to 150k/10 yrs in some US states). Possibly Toyota implemented better protection of batteries in pure EV mode or the batteries are better than expected to be included in HiHy. Europe doesn't get extended warranty on batteries so they do have EV mode on the Prius from the beginning.