Oddly, when I ask them why, the answers are mostly qualitative. It has nothing to do with "reaching the most people" or "getting exposure". It's a simple matter of taste. Designers tend to choose iOS products because they identify with them. They see Apple as an institution that they would enjoy being a part of in some way. Almost all the designers I know that are worth their salt use and love their macs for working in Photoshop and Illustrator, so it naturally follows that they prefer the iOS platform, regardless of numbers and market share.
Admittedly, my sample size is only around 50 or so, but this has been my experience.