Looking at the applicability section of 135, it basically goes back to a Rule from 1996. Non-transport turboprop 10-19 seats, Transport turboprop 20-30 seats, Turbojets 1-30 seats could be grandfathered into 135.
As far as I know, it is usually beneficial for the 135 scheduled aircarrier to go 121 for a multitude of reasons. I don't know all of them but I believe it has to do mostly with your relationship with the FAA/local FSDO and how you are set up to opperate.
Like MikeD said most regionals used to be 135. Just after deregulation in 78', almost all regionals were 135, but that was when they were considered air-taxis and local service airlines. With dereg, the regionals began code-sharing and the growth within the regionals was explosive. They quickly became feeder routes to the major and nationals. So the trend to incorporate them into 121 rules was almost inevitable. Additionally, I believe the a relationship between a 121 and a 135 is not as clear and therefore the push was to make all the regionals 121.