Because the voluntary behavior which maintained the status quo ante was observed to be changing.
> I think there is a battle over Netflix because its such an outlier in terms of bandwidth usage
While things like the attempt by broadband providers (largely also cable/IP TV providers) to impose anticompetitive discriminatory pay-for-access on competing video services were already on the horizon when the original FCC Open Internet Report and Order was issued (which is why such acts against competing video services were specifically called out in the Order in discussion of how its more general non-discrimination and non-blocking provisions would be applied), discrimination against particular edge providers isn't the sole focus of, or even really the main behavior change that motivated, net neutrality efforts. Blocking/discrimination of particular lawful end-user applications is at least as important in both aspects (of course, application-based discrimination and edge provider based discrimination overlap, since often an application is tied to a particular edge provider.)
> I don't think the ISP battle with Netflix can be generalized.
The issue, concerns, and specific behaviors at issue both predate the "ISP battle with Netflix" and have always been more general than it. Its not a matter of generalizing that battle, that's battle is simply today's most visible current manifestation of a general set of behaviors (Comcast's bittorrent blocking was the most visible recent exampe at the time FCC Open Internet Order.)
ISPs are suppressing Netflix not because of traffic asymmetries; the relatively small dollar amounts involved reveal it as an excuse.
Their real concern is that services like Netflix reduce the value of both traditional streamed TV content, and whatever value-add on-demand services that ISPs create and operate. Their worst nightmare is being reduced to dumb pipes, because that's a revenue growth dead-end.
At one point cable "on-demand" was a hot new technology that people were excited about (i.e. willing to pay for). Now people just get a Netflix subscription. Cable companies are not psyched about that.
I don't see the apocalyptic future neutrality advocates envision. I think there is a battle over Netflix because its such an outlier in terms of bandwidth usage, and because their stance on peering was at odds with the accepted understanding of free peering (the "status quo ante"). I don't think the ISP battle with Netflix can be generalized.