1. People like business travelers or those with even minimal levels of status/benefits (who don't pay for checked luggage) don't usually preferentially check bags because luggage gets delayed, it's harder to switch flights when there's a weather etc. problem, and they have to wait at the luggage carousel.
2. Hard and hard-ish roll-aboards are a menace. Especially in a world of generally more casual dress, soft-side luggage would make overheads a lot more manageable--understanding that some people really can't use shoulder bags or backpacks.