Why didn't the disciples preach the finished work of the cross before Jesus was crucified?

Because the crucifixion and finished work of the cross had not yet been accomplished, there was nothing yet for them to preach as an accomplished event. Even after nearly three years with Jesus, the Twelve did not understand His coming death; it was hidden from them, and they feared to ask Him about it.

Les explains that salvation is by faith and faith alone in the finished work of the cross, but before the cross that work had not yet been accomplished. So the Twelve could not be preaching something that hadn't happened; they didn't. In Luke 9:44, shortly before the crucifixion and after almost the full three years with Jesus, He said the Son of Man would be delivered into the hands of men. But the Twelve understood not, it was hidden from them, and they feared to ask Him of that saying.

Why could the disciples not have been preaching the finished work of the cross before Jesus died?

They could not preach the finished work of the cross because it had not yet been accomplished. Jesus had not yet died. In Luke 9, He told the twelve He would be delivered into men's hands, but they did not understand; it was hidden from them, and they feared to ask Him.