What are the economic incentives that reward an ISP for preventing access to portions of the web?

One simple exmaple would be to force people or companies to buy special subscriptions to access their preferred websites.
If the ISP market were truly free, that wouldn't be a problem since other ISPs that didn't do that would be more competitive. However, the ISP market isn't free at all and is controlled by handfull of major ISPs.

-another brutal example would be that some ISPs are also owners of streaming services. Abolishing net neutrality would give them right of life and death over their competitors. The most flagrant example of that would be time warner and comcast. What stops them from killing netflix? nothing. This isnt just theory... they actually did that, in 2013!!, when net neutrality wasn't in place. Every online business could be extorted into paying more so that customers can connect to their website.

Taking down net neutrality would mean to transfer power to dishonest and monopolistic corporations that will do anything for profit, as they have shown times and times again. The lack of competition and the dishonesty of those corporations is why net neutrality shouldn't be touched. You say that we shoudl ask ourselves who would benefit? what does the government benefit from net neutrality? nothing. The consumers benefit from it, having the freedom to browse the Internet with no restrictions or control from corporations. The online businesses benefit from it since it garantees everyone can access their website freely. Who benefits from stopping net neutrality? No one, except greedy ISP corporations that have no competitors and that have proven how dishonorable they can be.