One thing GnuCash doesn't currently do, but this Bigcapital apparently does, is inventory management.
Businesses large enough to hire one full time accountant will struggle to use GnuCash, and management (i.e., the owner) isn't going to re-organize all business systems around using an executable. They are going to migrate to Quickbooks.
(Note, however, it doesn't require a "web app" as it could just be a locally hosted database, but GnuCash does not support this feature.)
I have always found this frustrating because Quickbooks ~2005 has pretty much every feature a business actually uses, so it is at least theoretical that a "feature complete" open source small business accounting software could be realized.
Most of the discussions I see in forums here and elsewhere seem to revolve around web-based or technology-based businesses, but the bulk of the Quickbooks small business customers aren't any of these, and I would consider them as the target customer.
(GunCash is great for independent contractors and personal finance, so I'm not knocking it.)