That's a good question. I can think of a handful of reasons, but I am sure there are more. 1) ease of use. It's much easier to tell a prospective customer a URL than instructions on how to download/install/run a local application (if they even have permission to do so.) 2) Ecosystem. If your customers are generating content that needs searching, sharing, or collaboration, it has to be on the web in some capacity. 3) (Unfortunately) Shiny object syndrome. Browser-based applications have an innate appeal that outmoded desktop applications lack.