There was more competition up front at the time - Durie, Fleck, McInally, McClair (stop laughing), Johnston & McCoist (almost) all had justifiable claims to a starting place.. even McAvennie with his goals at West Ham & Celtic would have been a solid squad pick if he hadn’t black-balled himself from the international setup.
Duncan Ferguson’s emergence in the early 90s was another solid option.
Roxburgh seemed to ignore what most fans could see though - McCoist & Johnston was a great partnership, which produced goals for them and others in the team.