BC it’s easier for the any dev to package their program for flatpaks assuring it’ll work in all distributions, otherwise you have to wait for your package manager maintainer to repackage the program for your system. Which is what happens for Arch, debian, Suse, Fedora.
It’s not Thunderbird/program responsibility if they decided to make flatpaks the main source of distribution yet you decide to install it through other means. Which idk if they did but more devs are opting to distribute through flatpaks.