It costs lots of money to digitize and host the old stuff. And the non-profit organization doesn't lend itself well to selling access - it takes time and effort (read "money") to research with the lawyers whether "selling" or "membership fees" or whatever other business model is legally acceptable under the organization's structure.

But since new content is shot in digital, and edited digitally, it's a minor step to convert for streaming and also broadcast so they just stick to that.