During the early 20th century you'll find a huge range of button materials. Celluloid, glass, wood, bone, horn, Bakelite, and much more. A rubber button could be poured into a mould of any shape and design and would hold up to rigorous wear and washing. Something many of those other options would not be able to do. Plastic has it's boom later in the 1940s for uses like buttons, holding up to the same abuse.