There are many, many reasons.
One somewhat pedantic difference is Netflix deals with evergreen content. Any bundling of print journalism (especially with ISPs involved) is going to look closer to cable company bundling. Personally, infrastructure providers should not be in the business of selling content. I won't rehash those arguments here.
So, the ISP aside, the question becomes, who should you pay for a subscription to print journalism? Note that you already pay separately for a Netflix subscription; there would be no difference here.
Content is already bundled at the site level, eg a NYT sub gets you all the NYT articles. If we were to go up another level, eg buying a combined sub to NYT + WaPo + WSJ + Buzzfeed + HuffPo + <tabloid-blog> + etc -- then people would be complaining about the opposite, paying for content that they never read. Which is effectively the complaint that many people have with cable television.
The open question for these sites is is whether lower per-subscriber revenue, from a third-party bundling service, would altogether be greater than trying to solicit subscriptions on an individual basis. That might be the case, but sites also certainly don't want to straitjacket themselves in the manner of HBO, where bundling is their only form of allowable distribution.
Note that most cable networks would never be viable with an a-la carte model: it is only through massive bundling across millions of users that the smaller networks can achieve enough revenue. And not everybody can be HBO. That effectively rules out micropayments for most sites, and I still have my doubts that micropayments would even be feasible at the scale of the WSJ.
More importantly, that would be a major blow to net neutrality. I really don't want my ISP to be negotiating deals of what news I'm able to access.
I continue to use Internode almost solely because they provide a service I want. I've never heard anyone claim this is a net-neutrality issue.
If ISPs provided bundled premium / membership access to other sites I'd see that as a differentiating factor not a net-neutrality issue. So long as they weren't intentionally throttling other services, but they we would expect their bundled services to have higher QoS priority otherwise we'd complain the 'free' content was shoddy.
Personally, I view the whole net neutrality saga as a restriction on the freedom to enter in to contracts. But I'd probably change my mind as soon as it affected me personally. Biases hey.
Blendle (blendle.com) seems like it's aiming to be that
Articles probably make less money per person, but it's less customer hostile and I bet in the end makes these original content creators more money.
Jaron Lanier promoted that same basic scheme in You Are Not a Machine (2010).
It would also eliminate much of the incentive to bypassing paywalls.
EDIT: Apparently I'm replying too fast. Note that I quoted Netflix, implying similarity and not identity. I can also see the potential problems concerning Net Neutrality, but I don't think it has to be a problem. If done properly, no throttling is needed as the news sites can still serve ads to non-paying customers. You're just distributing your money in 1 place instead of 10. It's solely for convenience.