I always wondered about this when I race in Seattle.
In general it seems like people (racers & promoters) are antsy to get racing going by the time spring classics in Europe start, for whatever reason.. but yeah it would make sense to wait until late June, at least in Seattle.
The other part of this is for national-level racers, they can't wait until June to start racing. I guess it's up to them to travel for races but that's probably part of the pressure.