You'd be surprised how little people care about stuff like UI, UX, when they consider using computers a distinct and separate part of their job. I mean, I'm talking engine mechanics here and (to abuse a stereotype here a minute), middle-aged Southern Trucker Wives. The phrase "i'm not a computer-person" comes across the phone to me about 3 times a week, at least. We here are part of the internet cohort, and spend half or more of our waking hours on a computer. These people do not. And I know this because I'm wrist-deep in their aging, crusty, virus-ridden workstations. People that care about their computer time don't put up with this crap. People that don't use it til it explodes.
Personally, I hate QB's cramped ass menus and poor use of screen real-estate, how it filters its lists. The fact I always forget to change a company name in the right place to avoid a duplicate name conflict. (Here's a thought geniuses, USE SOMETHING THAN A STRING AS THE PRIMARY KEY.) That stupid links page, ugh.
Also tells me there's a potential market there, which I'm terribly surprised someone hasn't exploited. It wouldn't be terribly difficult (in a relative sense -- it'd still take a lot of work, s'just the bar to pass isn't exactly high) to make something that's less of a train wreck than QB.
The real barrier to entry is certification. That's what QBs has going for it. It's been around a long time, is accepted as a legitimate income and asset statement by most state governments and the US government. (And plenty of international governments.) It's not an insurmountable barrier and they do have competition. But at their level in the market, they have put in the time and have the household recognition. Consider them the McDonalds of Small Business Accounting Software vs. that new, great burger place that opened 6 months ago downtown.
Like I said, there are other, higher-end accounting systems to manage businesses
like McDonalds. We only integrate with QBs at my company because so many of our customers use it. We recommend everyone use something entirely different. (Although that ****ing thing has its own issues, as I've ranted about before.)