DDoS-Guard and VanwaTech are just specialist providers that offer hosting to websites that very few companies would willing to work with.
It's not an oligopoly either.
Cloudflare being a monopoly, or DDoS protection market being an oligopoly is absolute bullshit.
>It's not like you could start up your own.
You can. Vanwatech you just mentioned was run by a little kid who was bundling testcookie-as-a-service with filtering services from a bunch of other small providers like BuyVM.
You can get something like this started for much less than $1k/mo.
>Who else is big within the DDoS protection market?
Just off the top of my head: OVH, Voxility, Level3, Akamai, Neustar, Path.net, Stackpath, Hyperfilter, Javapipe, Sharktech, Blazingfast...