Why is Schedule 80 conduit not available at home improvement stores?
The big home improvement stores have elaborate systems in place to determine what sells in each store and figure out what products will bring the best return with the finite space they have for inventory.
There are lots of products that are very good or necessary that won't get a spot just because they are slow sellers.  Other retailers / wholesalers / distributors are set up to inventory more variety in specific categories.  So a local electrical supply will have things that are not as big of a seller.