Publishers investing in game projects need to see a high ROI to support a Linux client. This means some combination of increasing revenue or reducing cost/effort of multiplatform development.
Developers often run more on passion and interest than accountant like calculations; I think making multiplatform development easy is key to winning over support from smaller programmer run shops.
I also suspect that developers are limited in how they can price their game when distributed elsewhere. I don't have proof, and the contracts are kept secret, but why else don't developers price their non-Steam games at a lower price to reflect the savings of omitting the middle man?