One if generic, the other is country specific.
.com and .net are both gtld, thus good for the whole world.
.co.uk, .us and .io anre cctld, thus country specific.
Segregating TLD makes sense, they are meant to understood what's behind the website.
I think that cctld should be a sub off of the gltd. just take the gltds and append the country extension to them. (so the UK for example would be .com.uk, .org.uk, etc..)
I don't know if it would be wiser (and necessary) to effectively leave the US with the gtld, or to try to transition all of them to a .us sub.
As it sites now, greed and stupidity have now pushed the number of gobal tlds to over a 1000, I would see that 99% of those should be removed, or them all fall back under the rules for the global tld issuance.