What Tate said. There's also the additional fact that slowly people are moving away from metal shoes which is a large part of a Farrier's workload--and a part that pays just a bit more. Farrier's backs go out being bent over all the time. I was fortunate enough to learn to trim my own--note: not everyone else's, mind you--and that's been fine. I considered going to actual Farrier school but I have to trim sitting down already and can only do two or three in a sitting then have to rest a few days. Days. So imagine an actual working Farrier who does 6 or more a day for several days. That versus the pay, versus the decrease in trade jobs. There you go.
Because being a farrier is taxing on your body, especially your back. Also, society is moving away from trade work, so there is a lack of interest.