Mormons believe that, in the early days of the church, their church's first president and prophet, Joseph Smith, received a revelation from God called the "Word of Wisdom," which basically outlines the substances they should/should not eat or drink. "Hot drinks" [1] was the original wording in the revelation, which modern-day leaders have clarified means strictly "tea and coffee" [2]. This does not include hot chocolate or caffeinated beverages like soda or even energy drinks.
As I understand it from the outside, it is now official Church doctrine that the particular prohibition that was universally understood to apply to coffee and tea but sometimes interpreted more broadly is limited (in the strict sense) to coffee and tea (the latter, I believe, in the strict sense of drinks made from Camellia sinensis), but that rather recent doctrinal clarification did not erase the fairly strong tradition that the prohibition in application should be treated much more broadly than it's express limits, even if the Church does not specifically dictate the precise bounds, and so the widespread (though not universal) practice of abstention from caffeine as a stimulant prohibited by the broader prohibition in which the specific prohibition now deemed to mean “coffee and tea” continues.
This isn't inherently regional, though variations in the practice probably have some loose correlation with geography.
[1] https://www.lds.org/scriptures/dc-testament/dc/89.9 [2] https://www.lds.org/topics/word-of-wisdom?lang=eng