Eg. What could be the hidden angle of providing domains at cost ( which they do).
I think it's a matter of having a more complete cloud suite and expanding their portfolio while gaining market share.
Clarification: I don't think they do this at cost. Just cheaper than the big 3.
They seem to learn a lot by centralizing a lot of the internet's traffic through their core. Perhaps it makes their ddos-protection more robust when they can train on "what normal usage" looks like "for the longer tail".
Perhaps they are building moats around that business and expecting a future when Fortune1000 requires their cdn.
Their CDN is based on their SDN. Which is also at the core of all their products, including DDOS protection.
Their primary product is the SDN and a CDN was a logical pick as first product "release".
Eg. You shouldn't consider their products in "regions" as traditional cloud providers. In cloudflare it's programmable, not configurable.