It may have something to do with distribution rights.When Taggart was first released on dvd & vhs it was by a company called Clear Vision.The recent dvd releases seem to be from a company called Acorn Media.
There was a boxset released by Clear Vision a while back.It only contained the first 55 episodes.So perhaps Clear Vision only had the distribution rights up to then.
I'm sure after Clear Vision the dvd's were distributed by a new company