They are just not as good as QuickBooks and not many accountants are familiar with these other packages. The prices keep increasing but the QuickBooks online functionality is the best out there and it is the platform with which most other tools make sure to integrate.
One would think it can't be that hard to put together a webapp to freaking sum up some numbers, but nobody seem to want to do it in open source.
It could be because it's indeed so easy that it's boring, or could be that anyone with the necessary knowledge of accounting standards makes way more money doing other stuff than coding for free.
I'd assume that the overlap between capable open source developers and people who enjoy accounting enough to write their own accounting solution is fairly small.
