First, Quickbooks is frequently not chosen directly. Many, many small businesses choose their bookkeeper first and use whatever they use. One trick Intuit uses both in Quickbooks (bookkeepers) and TurboTax (CPAs) is to put a lot of effort/money into focusing on those relationships knowing that they have a huge indirect impact on acquiring and keeping customers.
Second, the fact that Quickbooks is so awkward and inconsistent to use isn't a UI problem, it's a feature. This is an important lesson in designing software that people use for their jobs. When you make elegant, intuitive software that almost anyone can use in short order, it stops being an impressive item to put on a resume. And it stops being a barrier to entry for competing professionals. Software like Quickbooks that's hard to use correctly becomes a selling point for bookkeepers and almost accomplishes the same purpose (though to a lesser degree) than professional licensing organizations...it limits the competition and keeps the rates they can charge high.
So many Quickbooks competitors have failed to knock them off their perch by not realizing that their customers aren't really their target audience and what should be their target audience doesn't want easy.