Re: make them ALL available AFTER broadcast as well - FOREVER
It's probably not the BBC at all. A lot of the stations' current output, especially in documentary, is commissioned from independent production companies. Rights to these films revert to those production companies after a certain number of exhibitions or after a certain time has elapsed.
This allows the Beeb to commission more programming, because each programme is cheaper to rent from a production company than to buy (make) itself.
The downside is that sometimes the production company can sell the programme exclusively to another owner after the BBC's use of it expires, which makes it near-impossible for the BBC or anyone else to re-acquire it, despite the public thinking that because it's on the BBC, it was made by the BBC.