Not OP, but also Mormon and served a mission with the same constraints. The biggest reason for this was that the two-year mission is seen as the one window of

Not OP, but also Mormon and served a mission with the same constraints. The biggest reason for this was that the two-year mission is seen as the one window of your life when you dedicate all of your time to serving God. Before that and after that you live normally, but during that window you're as close as the LDS church gets to being part of a religious order of monks. You theoretically chose to go out there (though to be fair there's enough peer and parental pressure for young men that some don't feel there was a choice), and you're with a companion who also chose to serve. The concern was that too much time spent on communicating with and dwelling on home would distract you and your companion from that singular focus and waste the preciously short time that you have before you come home and get caught up in normal life. Back in the 70s these constraints wouldn't have been a very big deal—most kids living away from home for college would have been in a similar boat, just less structured, and people expected communication across long distances to be slow and sporadic. Few would have paid for weekly long-distance calls in any case. But the church is pretty slow to change, so the rules stuck around longer than was likely healthy even as expectations around communication shifted with the advent of the internet. At some point in the last 10 years they realized that the slow communication pattern is actually not healthy for kids who grew up with expectations set by smart phones, and the formal restrictions are all but gone from what I understand. We video called with my brother every week in 2017-2019, and many missionaries these days are on Facebook or similar daily.