[Keith]

Fundamentally and bleakly, it's all just accounting. Drama is expensive; continuing drama less so, but still costlier than non-drama replacements. Continuing drama also suffers from having less commercial return potential, so it's not as worth the investment, especially when ratings are no longer standouts. 

Nonetheless, I suspect the cut in new non-continuing drama hours/costs over the last few years is roughly similar, it's just less noticeable as it's not in a permanent slot.

I imagine that long-running cast members in continuing drama can also become rather expensive. I seem to recall the first year BBC star salaries were published it included a few such actors/actresses, and then (perhaps conveniently) BBC Studios became private and therefore excluded from the list.

Presumably continuing dramas also have limited opportunities for being repeat on BBC, let alone being reshown commercially elsewhere. Whilst dramas probably cost more per episode to make they can be repeated multiple times on the BBC, as well as potentially being sold to other channels/countries/platforms.