Because the BBC needs to show it is attracting audiences!
Also not just the rating wars with ITV etc., but it creates a buzz if people are tweeting or gossiping about it.
Most things I prefer to watch week by week as I think you get more out of it. Binge watching means you lose concentration - or at least I do.
I suspect the ones they do make available online are the ones they think wont get a big audience, so not much point hanging on.
Also if they release it online at the same time as they are showing it week by week, more likely that someone will spoil it for everyone else by giving away the ending.
That's one of the reason I dont follow some threads on here as too many people assume everyone is doing what they do, ie binge watch!
I like the threads where you discuss episode by episode.
(But I do record series on commercial tv so I can skip over the ads!)