I'm thinking the companions/recurring characters probably don't sell that well. There were only a few Roses and Marthas, and one Donna. Jack and Sarah Jane had a bit more but they of course had their own shows-and K-9 usually comes packed with a Doctor or companion. Plus I don't see the Peri/Sil set being that popular.
The major draw of the line seems to be the Doctor and the recurring monsters. Although the season 5 line did have some 'huh?' figures, although not to the level of Grandma Connely or destroyed Cassandra.