From deals with ISPs that benefit from the caching of the traffic. The whole world fetching stuff from us-east-1 is an inefficiency that Cloudflare fixes, so the more customer traffic Cloudflare can cache or generate directly from its edge network, the better deals it can make with ISPs.
The peering and co-location partnerships also have a secondary effect of expanding Cloudflare's network, which allows Cloudflare to sell services on top of that, at large scale, with low latencies.
The peering and co-location partnerships also have a secondary effect of expanding Cloudflare's network, which allows Cloudflare to sell services on top of that, at large scale, with low latencies.