Because it's free.
The business goal of Trello is "100 million users, with 1% of them paying $100 a year and the rest paying nothing."
That means we need a lot of users, and to do that, you have to be horizontal!
The other alternative would be to pick a vertical (software development? Sales pipeline?) and make a product for that, and charge everyone.
We've already done that in the past (with FogBugz), and it's not a bad model for a small company, but once you have the resources to do things BIG, you should probably do them BIG.
Joel Spolsky spoke around this briefly at the Trello launch. A partial transcript and video recording is here on the techcrunch site. Essentially the idea is for Trello to be used horizontally as The project/work flow manager, not a developer specific tool like other Fog Creek products like FogBugz.