Because everybody would only watch the old stuff.
The new stuff is useless.
And then, how would the institution pay for its employees work program if people were only watching the old stuff.
Most people at the BBC could be fired with no loss to income, if iPlayer was unlimited in content.
This is why iPlayer doesn't give you the old stuff anymore. And why the old stuff is never shown live either.
Yes, they give you an Archive choice but this is heavily limited to upholding the work program. And is missing the art.