Why don't Muslims convert to Mormonism, since Joseph Smith is an even more recent prophet than Mohammed?

For the same reason all prior religions don't convert to the new one: Smith was a false prophet. Not to mention, have you ever sat down and read some things about Mormonism? It gets a 10 out of 10 on the ridiculous crap meter. Plus, it's too new. People have a tendency to ignore ridiculousness and revere something as sacred the older it gets.