One reason for having researchers teach is that in specialized subjects, it’s hard to find people who know the field well enough to teach it that are willing to be full-time teachers instead of actually doing work in the field (and keeping up with new developments, etc.). This probably matters more at the Master’s degree level than the bachelor’s degree level, though; there are a lot of people who aren’t Richard Feynman who can teach undergraduate students how to solve the Schrodinger equation for the hydrogen atom, but the number of people who can teach string theory to physics graduate students is a lot smaller.
One reason for having researchers teach is that in specialized subjects, it’s hard to find people who know the field well enough to teach it that are willing to be full-time teachers instead of actually doing work in the field (and keeping up with new developments, etc.). This probably matters more at the Master’s degree level than the bachelor’s degree level, though; there are a lot of people who aren’t Richard Feynman who can teach undergraduate students how to solve the Schrodinger equation for the hydrogen atom, but the number of people who can teach string theory to physics graduate students is a lot smaller.