You aren't guaranteed an app ecosystem just because you release a product running RANDOM_OS. If I were to release a line of laptop computers that didn't run Windows or Linux, but ran some in-house developed, incompatible-with-everything-else operating system, I can't demand that Adobe release a version of Photoshop and Microsoft release a version of Office for my platform. I shouldn't run to every software vendor (open source or closed source) and order them to compile and support a version of their products for my OS just because it exists.
If my custom OS gained enough of a following, then I might be able to cut deals to have more applications developed for it. Yes, this means it is an uphill battle to introduce a new operating system. Even if my custom OS were superior in every way to all other OS's out there, it would have a rough road to travel. It could easily fall victim to the chicken-egg problem of developers not wanting to support an OS with few users and users not wanting to switch to an OS with few applications. That's just the way the market works, though. You can't demand support for every custom and hardly used OS out there. Software developers don't have the resources to do this and demanding they do it makes no sense.