I assumed that there were space constraints. If I recall correctly, one of the reasons that Dealer's Row was instituted at Arisia many years ago was that there simply wasn't enough space in the Dealer's Room to accommodate everybody who wanted to be there.

That said, I had the clear impression this year that Dealer's Row didn't have as many vendors as in past years. My wife was looking for the henna tattoo person, and was really disappointed that she wasn't there.