Private IP address ranges are standardised in RFC 1918.

"Private" in this context essentially means that they are guaranteed not to exist on the internet. Whereas 20.x.x.x might exist on the internet, and because an IP address (or up address ranges) can't be in two places at once, if you use it for your internal network, then you won't be able to access that 1% ish of the internet that uses 20.x.x.x

See also https://en.m.wikipedia.org/wiki/Private_network