Hmm, good question. I don't know everything about all the sawmill styles, but the old fashioned circular style maybe wouldn't be portable?
At least with the bandsaw type, you can go to where the timber is, instead of having to haul everything to and from the site. This could make the sale more attractive to the prospective timber owner wanting to see his own lumber being made.
The band type turns less of the wood into saw dust also. The one pictured is capable of 3500 bd. ft. a day.