Walmart caters to the lowest common denominator, and also realize, WD media is rare.
The TV series dvds never got a huge release, the soundtracks even fewer. No company will make more copies when they cannot be guaranteed of making a profit. Simple economics. Yes it would be nice for them to re-release some stuff....Mike Batt took a huge chance with his own label that he would sell the Caravans/WD double disc.
Now what about downloads...get the rights to the music or film, and sell downloads...cheaper for the supplier right? Depends...I don't know who has the rights to the original soundtrack by Angela Morley...I think its Sony. The TV series is up to Mike Batt what gets done with it, and even then he may not have total control over the rights of the songs on the original disc.