there really is a lot to do and see around Plovdiv
Although I do remember an interesting old town in Plovdiv and the Valley of Roses nearby (been there many years ago), it's still virtually unknown place for Western Europeans. Probably that's why it will commence with just two flights a week.
That's a good example of how extremely prudent Ryanair became in launching its new routes. Even if Plovdiv isn't a famous place for Britons, one should take into account thousands of Bulgarians working in UK and elsewhere in EU. Twice weekly? Too cautiously perhaps?