In the help I found "The Crossfade option is only available if the end of the new clip is not over any existing content on the track."
But I'm not sure exactly what that means. In this particular case I group my full timeline together so I could insert a clip at the start. Since there are about 10 tracks I didn't want to have to manually move each one. When I try to move the entire group back, Crossfade is not available.