Bundling allows cable operators to segment the market: to sell the same product to different audiences at different prices depending on their willingness to pay.
A $100 package of bundled sports + movies can be sold to the person who is willing to pay $90 for sports and $10 for movies and also to the person who is willing to pay $10 for sports and $90 for movies.
But sports and movies are unbundled, they must be sold at the same price to everyone. Should they be priced at $10 each? $90? Somewhere in between? Pick any price and the cable companies lose money: the person who values it more will pay less for it than they would have, and the person who values it less won't buy it at all.
EDIT: I don't want to sound pro-cable; indeed, I am not; I am just pointing out the effects of this legislation. In some ways, bundling is a natural way to package content. MOG, Spotify, and other subscription music services bundle their content (you don't get to pick and choose what you subscribe to), and the big daddy of disruption, Netflix streaming, doesn't let you unbundle either. It's one flat fee for everything. In these cases, of course, it's easier to swallow because prices are so low to begin with. Perhaps it's not bundling that's the problem with cable, but the pricing.