Avalon went with a very simple chip on a large(110nm) process. I imagine this strategy gave them very high yields out of the wafers because of the small chip size. This came at the cost of a large PCB to route to all the chips.
Well, that's certainly going to be part of the reason. However, take a look at the distance between chips on the BFL board and compare it to the distance between chips on the Avalon board. Now remember Josh's comment about the PCB ground plane not being able to dissipate enough heat due to the thermal density of their design, and how having to redesign the chips to fix this delayed shipping whilst Avalon shipped on schedue using similar QFN chips.