Probably simple question and I miss something trivial but... Typical CDN incapsula setup is:
Why is A record used? Why don't use CNAME record for root domain also?
Because you cannot have a CNAME record for the root domain if you want to have other records under that same domain. This is not allowed by the DNS specification, specifically in RFC 1034 (http://www.faqs.org/rfcs/rfc1034.html)
If a CNAME RR is present at a node, no other data should be present; this ensures that the data for a canonical name and its aliases cannot be different. This rule also insures that a cached CNAME can be used without checking with an authoritative server for other RR types.
If you had CNAME flattening then you "could" do it.