Because Cogent and Comcast have a peering agreement in which they agree to exchange traffic that is more or less symmetric (since that means that both partners will be spending approximately equivalent amounts on infrastructure development and maintenance). It has been customary for consumer-bound traffic to not count against the balance, since it was a relatively minor portion of the overall traffic exchanged. With the advent of high-resolution consumer-oriented video services like Netflix, that has changed - consumer traffic has become a significant portion of overall traffic exchanged, which causes a flow disparity between the two peering partners and places a disproportionate burden on one.

If this were not consumer-bound data, then Cogent would likely already be paying for the disparity. As it is, it's a good guess that Cogent has refused to compensate Comcast for the additional load on their network build-out and maintenance costs, instead relying on the good-faith consumer data exception to exempt them from paying for it. Comcast's response to this was to steal Netflix's business from Cogent, thereby alleviating the uncompensated load that Cogent was placing on their network and improving Netflix's deliverability.

There's no such thing as a free lunch. If you're a service provider, you're paying someone for delivery of your bytes. Netflix just cut out the middleman here. It's not like Netflix didn't already have to pay for the bytes they delivered - they just changed who they're paying.