Because there are adequate channels for religion- there are chaplains for every faith, etc. Creating needless division and antagonism by trying to convert other people instead of respecting everyone's faiths (trying to convert someone is an automatic disrespect towards their faith, no matter how you look at it) adds more tension to an already tense environment in which those divisions can easily become toxic. Again, no one is saying soldiers can't profess their faiths openly or even talk to each other about them. But chaplains aren't missionaries (nor should they be, since they work for the government,) and soldiers shouldn't be, either.