I can only assume the studio hopes the reps of Denzel and Gary Oldman will pull enough people in the first week or two to make a little money. The film could not have been expensive to shoot.
As for why two fine actors like Washington and Oldman would sign on for this… no idea.
If you’re the type that makes a mental list of “reality errors” in the movie (where do they get gasoline 30 years after the end of the world?), don’t bother. The Book of Eli has too many.