No reason except the fact I don't like noisy fans. :)
Seriously speaking - it's just easier to find a case without a fan from what I see. But if someone find or design a good case with a fan - we'll use it.
lol I hear ya, Im just thinking with that seemingly minimal amount of heat (that many BTU's this would raise the temp of a us gal of water by 8.2f, or a liter of water by ~17c), it shouldent really make too much of a difference to have a rectangular case thats enclosed & has a few holes punched in it...
To my knowledge, theres no special cooling considerations taken into account for any of the USRP's?