The tricky part of this question is why they can't synthesize suitable food, as single-food dependent animals are pretty well known on Earth - giant panda, snail kite, etc. (Many of these animals do eat something else occasionally, but they are definitely totally dependent on one food species for survival. Similarly, the larval-food dependence of many moths/butterflies.)
-non-native technological development: the space travel technology doesn't come from a generally high tech base but was obtained from aliens or uncovered from the ruins of a previous destroyed civilization on their world.
-impaired biological development: something about them or their world makes it extremely hard to develop biology relative to the other sciences. It could be some misleading fact (introduction of alien species in the distant past or really high rate of radical mutations) leading to wrong theories of biology becoming dominant, or something cultural (a biological-weapons war almost made them extinct so genetic research is forbidden, long period of dominance of a world dictatorship pushing something like Lysenkoism).
-aberrant means: their space travel tech uses something like FTL or teleportation that doesn't fit well with our physics, and turns out not to require a high general level of technology.["The Road not Taken" by Harry Turtledove] Possibly their brains are wired fundamentally differently from humans', so something is obvious & simple to them that isn't to us.