Basically obedience is the reason ... we were told not to partake of hot drinks and that was later defined at tea and coffee and that's that. There is no massive secret to it, that's it, as I said there was a lot of members who used to believe it was about caffeine but officially that's not the case and other caffeine containing substances have never been called into question. Why those two drinks? No idea. I personally drink hot chocolate and another substance called barleycup, I avoid anything with tea leaves in including green tea too ... my husband drinks herbal teas such as elderflower, and 100s of other varieties as long as it doesn't have the tea leaves in it.
Alcohol, we are told "strong drink is not for the belly" we avoid drinking alcohol. I have no issue with it in food, many people cook with it but a lot equally avoid it ... I don't have it in desserts ... many will avoid things that look like alcohol whereas I have a massive supply of non-alcoholic drinks. I also socialise and have no issues being around anyone drinking or even having it in my home, we have a tidy supply of wine that we get given periodically and friends will often raid our supply for something. Other would avoid being a round it or having it in their homes.
Like everything, people all live it differently and understand it differently, not every Mormon is active, not every Mormon lives the word of wisdom, some take things to extremes and there is a massive cultural aspect too, like some Mormons will disapprove of facial hair on men but it really has no relevance to the doctrine and is such culture (my husband has facial hair).
If you have questions just ask them, or ask me. I'm fairly honest about the good, bad and the ugly ... I am fully aware of how weird we appear.
Do we live longer? There is some antidotal evidence to suggest maybe, but I've personal not seen any particular evidence of that. We top have leadership run by very elderly people, our prophet died today aged 89 and his successor is aged 93, but then I have seen many people die very young and cruelly just like in life in general. There is no doubt that smoking and excessive alcohol have an affect on your health, though, but as for tea and coffee not sure there is any particular benefits, I think it we all truly lived it, eating foods in season, having a great supply of grains and herbs, reducing meat, and getting adequate sleep it would have a greater impact in our overall health. However generally people often live the "do nots" rather than the "dos".