Images and Videos tend, in comaprison to text, consume a lot of data even if compressed. This additional data leads to a stark increase in bandwidth usage when accessing a video compared to, e.g., a (text) blog.
Because cloudflare is the one handling most of the bandwidth usage and routing this amounts to a significant cost for them (more servers, bigger pipes, ...).
All of the pipes should have been implemented by ISP's and Telcos if all that damn Federal grant money would have actually gone to infra instead of exec bonuses and lobbying.
Because the large carriers make a ton of money by extorting their clients (ISPs and data centers) with absurd data interchange pricing. Some like Deutsche Telekom are even worse, they charge both their residential and mobile customers as well as anyone trying to peer with them.
That's partially also the reason why the extremely large players like Google and Facebook have built their own sub-sea fibre lines. Utter madness.
I don't fully know, but I am guessing because Cloudflare doesn't charge for bandwidth, only requests... non-html content is going to be more data per request, which might not fit into their cost model.