Because, per Spolsky, Fog Creek wants to achieve a widespread adoption first before they start charging for it (with probably a free tier remaining). Other than that, Trello is very much a commercial app.
Fogbugz IS a commercial app, and at $25/mo (if I remember right) it's in the same category as most other commercial apps, i.e. it's not particularly cheap. It's still down.
Fogbugz IS a commercial app, and at $25/mo (if I remember right) it's in the same category as most other commercial apps, i.e. it's not particularly cheap. It's still down.