Most likely because they're scratching an itch and don't happen to use Firefox. This is similar to why a lot of projects on Github don't support Windows despit

Most likely because they're scratching an itch and don't happen to use Firefox. This is similar to why a lot of projects on Github don't support Windows despite the market share of Windows being much bigger than that of both Linux and OS X put together. Among the demographic Joe Randomdeveloper cares about — himself — Chrome on OS X has 100% market share.