Great question!
First if you want to be more informed on the general topic of information distribution channels in history, such as the telegraph, the telephone, the radio, satellite communication and the internet, I highly recommend Tim Wu's book "the master switch".

As for the argument supported by the content distributors such as Comcast & Co (ISP) well it's fairly straightforward.
They have been investing for years now in fiber optic cables around the USA and the world to ensure fast and reliable internet connection. It's probably important to note that these investment have often been made in private-public ventures where the US has invested lots of money. Now because in the current system (net neutrality) ISP have to open their fiber optics to anyone paying and cannot prioritize content transmission coming from their subscribers, they cannot ensure that all content will be delivered in time unless they always make sure there is extra transmission capacity in case their's a peak of transmission demand.

Until now, this has never been a problem because in the 90's, they invested so much in the infrastructure thinking it would be a goldmine that when the .com bubble burst in the early 2000 due to crazy investment in fiber optic, ISP's had to fill their lines to be able to pay for their investment. Now that they managed to fill them, they want to make more money without investing too much in infrastructure again because it's risky, time consuming, mega expensive (about a million a kilometer) and not as profitable as it could be without net neutrality. Another problem they are facing is that they are also becoming huge content providers, meaning you subscribe to an ISP and he will offer you TV programs, music, movies you can stream etc. When you pay for that service, they want to make sure you can stream without any buffering (because you paid for it right!) and they can't ensure that with net neutrality unless they always have a margin on the amount of information they infrastructure can handle.

Now this is where the real dilemma comes in: if you want to be able to pay for an ISP's content such as movies and don't want to have any buffering (which btw is almost inexistant today) then ending net neutrality would be a good option. On the other hand, if you want to subscribe to another content provider such as Netflix (if it doesn't belong to your ISP provider because they work with a competing company) then your IPS provider will not prioritize this content on their network and you will have a hard time watching the movie you also payed for! In short, this will tie you to your ISP, and if you don't have a choice of ISP's at home, then your access freedom to the market of content is reduced to what your ISP provider has to offer.

There is also a bigger fight at stake here. ISP's make their money by offering broadband and content to subscribers. The Valley make money mostly with advertisement. Advertisement is content that the ISP's have to "carry" which increases the broadband demand and reduces to IPS's ability to offer paying content to their subscribers, their market. In short, the Valley makes money off wide broadband offered by ISPs and do not pay for that as much as they should!

However, and this is my opinion, opinion you can find in the book i suggested at the beginning of this long post, it's not the right solution in this debate to let ISP's take control on the content distribution market by ending net neutrality. The harm caused by this change from the current situation would be much to great for users and the content provider market then the benefit these ISP would make. They arguments are valid but the solution proposed is flawed, especially considering that in the USA, you only have 4? ISP providers left for a 350 million american market. I believe our liberties are already reducing in terms of the information we are exposed too, it's not easy to find what you need out there (as you may have seen when searching information for this post) and we should not give the chance to huge companies or governments to interfere anymore because they may (and will) use it to their advantage when the time comes... and that the precise moment where We will need it the most!