Dropbox without using http_port_80 would only work in a laboratory. One big reason Dropbox is valued at $1+ billion dollars is that it uses http so everyone can use it easily. Web browsers didn't require radical changes. Corporate firewall rules didn't have to change. That's how they got quick adoption and millions of users.

A business decision to use a specific network protocol doesn't define what web and network protocols are.

It is is just a consequence of what network protocols happen to be open by default.