To start off with, you might want to look at https://www.ubuntu.com/download/flavours and https://wiki.ubuntu.com/DerivativeTeam/Derivatives to see that Linux Mint really is not a official Ubuntu flavor.
However, the underlying reason is that while Linux Mint's codebase is Ubuntu, it does not use the Ubuntu repositories. The Linux Mint team changes software as they see fit. In the same way that Ubuntu uses Debian as its codebase, but Ubuntu isn't a flavor of Debian, Linux Mint isn't a flavor of Ubuntu.
Linux Mint has no Ubuntu branding or affiliation (other than using the codebase) with Ubuntu.
While it is similar, if you look at the nitty-gritty of the repositories, you will find that Linux Mint is very different to Ubuntu.