I agree with FrostWorks, its all a matter of having the time to ensure the build is of quality for the users.
Imagine selling a game for £10, and you exported a build for a platform but couldn’t test it, so you just uploaded it as-is. Now imagine that the engine has a feature that is not supported by that platform, so when you compile for that platform, it tries to do a workaround, that doesn’t look as you’d expect, and now your game looks glitchy.
Now you have to deal with customers that spend £10, and are not satisfied by the quality you delivered.
That’s at least one of the reasons. I don’t have numbers, but I’ve heard some platforms are not worth (financially) developing for. I like to think that most indies don’t think like that, but I can understand those who do.