Two reasons for that-
1. End of WWI left Du Pont with an over capacity for Nitro-cellulose, the basic material for explosives and Celluloid, but with a little market demand.
2. Until then, Celluloid was not considered as a material by itself, but as a substitute for Ivory etc.
So, after end of WWI, Du Pont started promoting celluloid as a "new" material.