Spring Break is also earlier this year for most places due to Easter being at the beginning of the month of April. Colleges are earlier, and school aged kids are earlier (we are going the last week of March for instance because of our Spring Break).
With that being said, Poly still has good availability, and so does SSR, and it seems like the only week really fully missing dates across all resorts is the 14th-21st (a lot of college spring breaks starting then).