Why so?

There is a lot of darkness, and we can only look in so many places. We tend to look along the plane of our elliptic because that is where everything in our solar system is. Looking elsewhere is possible, but increasingly lonely/low odds.

I don’t know if that applies to the sky surveys.

1) they are new 2) they are still limited in coverage area (https://irsa.ipac.caltech.edu/applications/FinderChart/docs/...)