The reason is probably that the market for blog editors disappeared when WYSIWYG editors became good enough for most people and wifi became common. The level of hassle "fat clients" in the space end up being (with crappy caches, blocking interfaces etc) is not offset by the tangible benefits of offline editing and marginally-better media support.

I'm not a great fan of the Blogger editor, but I've not found any modern alternative that can beat it so much to consider paying for it.