I have wondered about this as well but have never seen any real explanation or investigation into the situation. When tallow started to go out of fashion and seed oils mostly took its place, where did that stream of tallow go? Discarded, used in other food products, industrial use, animal feed, other countries?  

While tallow is making a comeback or sorts and is big on certain corners of the internet, pro animal fat/anti-seed oil eating is still fairly niche in the real world. It won't happen overnight but markets can adapt and change over time to make it happen if the demand is there.