Educated guess - at the time of the inception of the first of the Falcon trijets, the Falcon 50, the only proven light fan engine was the TFE 731 series which had a track record on the LR35/36 and 125-700 (the later 731-5 series was fitted to the Falcon 900 and 125-800).
The only other suitable contender in the power bracket for a 2 engined alternative being the ALF502 from Avco Lycoming which at the time has all sorts of teething problems which were being experienced by operators of the 146 and Challenger 600.