When we started out Adobe was still sticking to their Flash guns. As HTML5 was the only method to make animated web content on iOS, it was clear someone needed to make tools aimed at graphic designers, so we decided that someone should be us! Luckily with no other competing apps around, we didn't have to deal with an uphill battle on feature parity unlike these vector editors do with Illustrator. So I wouldn't say we were disgruntled, just trying to fill in a hole. And, on a personal note I had a need for such a tool for a side project I was working on.
OS X is a great platform to develop for generally speaking. We naturally leverage WebKit, and Quartz and CoreImage frameworks I'm sure do a lot of heavy lifting for the vector-based tools. Also keep in mind a larger percent of graphic pros use a Mac than the general population, so if you're building a tool it is a safe bet to start on the Mac.