1) The Yak40 is NOT a corporate jet. It is an airliner that seats from 24 to 40 pax depending on config.
2) It was designed to operate from semi-prepared (gravel) or grass airfields.
3) It was designed for internal Russian feeder services, hence it often operated multi stop, bus like route from very small airports, hence having built in airstairs was convient
4) Built in airstairs were once very common and can be found on the Caravelle, B727, DC9 and B737-200 (not rear airstairs), also BAC 1-11? & HS