The why is that there was a huge solution/market gap.
Finding software for Palm and Windows phones was often a trek through random forums and websites to download garbage that didn't work.
There needed to be a central registry before smartphones would be adopted by the average person, and the registry needed some quality assurance so that the average person would have a good experience.
Tucows wasn't going to cut it, and the only open model that worked at the time was linux distro package repositories, which would need a dozen features tacked on before it was usable, and only added packages glacially (that QA problem again.)
Smartphones wouldn't be a thing without centralized app stores, but I'm also in favor of allowing users to add third-party app stores, in the belief that 99% of people won't do it, and the 1% would understand the security risks.