Earlier quoted context omitted.
Because the ISPs control your Internet access. They don't compete with each other. Without net neutrality, they can prioritize, slow down, or block any site or content they want to. Many people cannot simply switch away. They have only one ISP to choose from. You can switch away from Facebook whenever you wish.
Next logical question: net neutrality has existed for a very short amount of time. The Internet & Web did extremely well before net neutrality, with few instances of abusive carrier restriction over the prior decades. What is the very strong supporting evidence to demonstrate your scenario is the likely outcome? If you want to regulate something so important on the premise you're suggesting, you need to present equal…
https://www.cnet.com/news/fcc-formally-rules-comcasts-thrott...
That's effectively what lead to the situation we are in now. FCC said it's illegal, appeal ruled FCC doesn't have the authority to regulate at the time, FCC makes attempts at making "net neutrality" a thing they can control, and now we are on the verge of having them roll it back.
Basically, ISPs starting pushing the boundaries and a war directly against the concept of net neutrality and that fight is the one we are currently talking about right now, this is just the latest battle.