FamiliarityI've released many other Open Source programs under the New Breed Software name.
SupportIt's easiest to get help from other developers if they can see and manipulate the code. (Most are quite familiar with Open Source licenses, as well.) There's absolutely no worries using Open Source libraries and tools, too!
CompetitionCommercial software is a big market. So is Shareware. How could I compete there? As a hobby, the project would be doomed to failure without a userbase!
Karma, RecognitionI get paid to do development at a big corporation. I get 'warm fuzzies' and world-wide recognition doing Open Source!