There's just not that much red tape in Australia. Starting and running a company costs a few hundred bucks per year. If you keep up your books, reporting takes a few minutes per quarter.
Bam, you've successfully navigated all the red tape needed to run a software business.
For corporations, tax rates lead to domicile-shopping because finance analysts use Earnings per Share (EPS), which is computed after taxes, as a key measure of company performance. Relocating from somewhere in the U.S. to the U.K. might substantially increase the total taxes paid by employees and executives (almost every developed country has a higher individual tax burden than the U.S.), but it can goose that all-important EPS number.
So Australian companies tend to pay dividends much more freely than US companies, but the headline corporate tax rate is higher.
Tax would be a big part of that. I expect that American familiarity and comfort with UK companies would be another.
Lots of Australians would qualify for permanent residency in the UK under an Ancestory Visa, or an equivalent in another EU country. Emigrating AU<->UK is a very well worn path that's easy to naviagte. The US isn't quite so welcoming.
(Source: i'm an Australian who live in the UK for 10 years, the US for 2 and now Canada for 5.)
I tried to buy NZ stock exchange shares in the UK, and my broker didn't cover it.
Not sure what the situation with Australian ones is. But basically, the more brokers abroad that can get them, the less the advantage of being on NASDAQ.