iso639 is compatible with an older version of pycountry, so it makes sense to use the more recent version to be more up to date.
iso639 had its last commit more than 7 years ago, leaving the package seemingly unmaintained. According to their classifiers it does only support python3.5 or lower, meaning that it officially doesn't support any maintained python version. While you can still install that package via pip, modern package managers like poetry look at the compatibility information and therefore reject the installation. Hence, dropping iso639 would allow checklist gain greater support by dropping iso639 in favour of pycountry.
use pycountry over iso639 for 2 reasons: