I think the problem is that tiling window managers have never been mainstream, so they are all targeted at people who would go out of there way to install a tiling window manger. By contrast, even non-mainstream composting window managers have the benefit of seeing how the mainstream ones do things to be usable (and have had more effort put into them in general). Another problem is that tiling window managers are (almost?) never part of a complete desktop environment. They may pay lip service to this (in the form of a basic start-menu type thing), but for the most part they are simply window managers. With composting window managers, no one (except the developer) thinks of the WM by itself.