It's all of that plus the cost of the servers, electricity, employees, etc. I'm not sure if they use AWS, but they'd definitely be using a premium blend of upstream providers (no second-rate providers like Cogent, Psychz, M247, etc) to avoid latency and reduce buffering, which I'm sure is what AWS does for their video streaming service too.

I wouldn't be surprised if Twitch hit egress bandwidth usage in the order of terabits per second during peak times. Having that much bandwidth available is not cheap.

The tricky thing with streaming is that the data can't be cached, since it's streaming in real-time, so it'd cost way more than stored on-demand video like YouTube. Big companies like Netflix, Facebook, Google, etc give large ISPs some of their caching hardware (usually for free). It significantly reduces bandwidth costs for both the ISP and the service, as YouTube and Facebook are often over 50% of an ISPs bandwidth usage.

If you look at a popular YouTube video, or a popular video or image on Facebook, it's likely coming from within your ISP's network, which is practically free for them. The caching box only needs to download it once from the upstream servers. None of that is doable with a live stream.