Most Mormon missionaries follow the rules set out for them. Limited exercise time and have to stay with your "companion". Work too tiring to really train anyways, etc.
The question we should be asking is why don't the Mormons update their missions for the 21st century? 40 hours of missionary work a week and being able to go out and explore, integrate into the local culture, and keep fit would probably do better to spread their mission. The companions could be more like roommates instead of butt buddies. They have been really unsuccessful at getting converts for quite some time.
The Mormons are more active in Brazil than any other country, but despite that less than 2% of the population is Mormon (2% is Christian other than Protestant or Roman Catholic, a differentiation which Mormonism is lumped into).
Next up is the Philippines, less than 4% other Christian - again Mormonism isn't even big enough to be included seperately.
Nigeria? 0.6% other religions - a category that includes all non Abrahamic faiths and everything else.
Even in England, where there isn't much of a cultural barrier, they fail.
Instead of sending these young men out on missions and stunting their development for 2 years, they should be encouraged to use their own resources and supported by the church to spread their faith after college.
You are going to argue that missionaries do other work that benefits the community, not just proselytize. That's true, but the work could be better done by sending mormon professionals to teach the locals how to improve their communities and giving them the money to do so directly instead of supporting young people who have to give up 2 years of their lives.