Presumably the big scary sysadmins have access to the *.cloudflare.com DNS records, and marketing just needs to push this thing right now and can't wait, so it's easier for them to buy a new domain with a shiny new TLD than wait for pay.cloudflare.com to be authorised.

cloudflare.com/pay probably has a similar chain of approval: if every marketing idea had its own top-level route, it would get pretty crazy with such a big company.