There is no good reason the CBC cannot approach the copyright holders of Creative Commons works the same way they approach the copyright holders of commercial works. In fact the CBC is much more likely to obtain permission without having to spend money in the case of CC-licensed works. There's no good reason why they would have to completely avoid this option.
There's no good reason but there is a reason. There is a lot of money and political clout behind the monied copyright interests. This maneuver benefits them, satisfying "que bono?". The question is whether you think that's a complete and total coincidence caused by a completely free, unprompted, un-coerced, un-pressured decision on the part of CBC. Anyone who wants to believe that so badly that they'll dismiss all other notions as "conspiracy theory, get a perspective" is either naive or deluded.
I've got another reason for you. Time and money. Consider this.
Negotiation for license rights = 50/hr (numbers pulled out of my ass. This includes the negotiations, contract signing, accounting to cut any cheque for payment, time for the legal dept to review the contract, etc)
So, to license from a library of works = 4 hours of work.
To license from 750 different license holders = 3000 hours of work.
Hmm....$200 in labour for a blanket license, or $150,000 to license 3 songs / podcast (5 shows a week + 2 weeks vacation/reruns in a year. Add in the costs of researching and maintaining records of each license agreement....). As long as the cost of the exclusive library license is less then the cost of negotiating all those individual license agreements, CBC will go exclusive.
Could they go and use CC music? Yeah, no reason not too other then the cost.