The answer is simple; it's a slippery slope, and one that won't be easy to clamber back up. Changes to freedoms on the internet for any country set an established precedent for other countries to do the same, and if the USA changes these laws then it's only a matter of time before other countries do the same.
Net neutrality is a simple concept, and one that many people believe is just an unarguable, inherent fact of the internet; the freedom to visit any website freely. Now there are a few long-standing exceptions to this hard and fast rule, but those outliers are for public safety and legal reasons. For example, some websites aren't allowed in most countries as they are for illegal activities such as purchasing illegal drugs, or the services of an assassin on browsers like Tor using .onion instead of .com or similar. These sites are illegal for good purpose, and it is rarely argued that people should be allowed access to these sites. But, despite what some obfuscating people would like the public to believe, is not what net neutrality is about. Net neutrality is about keeping the full, legal internet available to anyone with an internet connection.
The analogy goes like this; say you pay a certain amount each month for a library card, because you like to use your local library. Now, you wouldn't expect to be able to rent a book on home bomb-making, because it's likely to be used for illegal criminal purposes. But say you wanted 3 novels from separate authors, and you got to the counter, scanned your card, and suddenly you can't rent them all. The librarian is a big fan of JK Rowling, and lets you take the book for free using your card. The second author is Stephen King, who she believes is too dark, but she lets you rent it anyway for an added charge. Now the third author, George Orwell, is one she ideologically detests, so she takes the copy of 1984 and does not let you have access to it. Now this is clearly unfair, because the librarian is imposing her own ideological beliefs on your choice of media, and is censoring the library.
This is exactly how Net Neutrality works; the library is replaced by the internet, and the librarian is replaced by your ISP. Neither the librarian or the ISP has any right to dictate what content anyone receives, as long as it's not the outlier illegal content for inciting hatred. But if the current laws on Net Neutrality get repealed in America, then the internet will no longer be a place for the free exchange of information and ideas that it is today, it will be a politically driven censorship of individuals and a direct violation of freedom of speech.
Imagine this; you live in one of the many areas of the world with only one service provider, and are already being strong-armed into paying through the teeth for internet access. There are no other options, if you want internet access, you go through the only choice. Not ideal, but you can deal with this, albeit reluctantly. But what if this service provider was one with varying interests, such as Virgin Media. If net neutrality is repealed, then Virgin could block access to competing websites, such as Sky News or holiday websites. This type of blatant censorship would become common worldwide, because strictly speaking, it makes sense business-wise. People have a choice, either censored internet or no internet, but that's not really much of a choice is it. Politically driven access to 'free media' is oxymoronic, and 1984 wouldn't be the first radical thing to be banned. It's bad enough that these ISPs are throttling data usage across the board without extra payments, but to censor the one place on Earth for truly free speech? 10 years ago this would have been laughed at, yet somehow it's happening.
Service providers have already been throttling people worldwide, do not let them choke us too. Keep our net free.