I'm from Uruguay and I've also always wondered why heroin never arrived. The people you would consider junkies in Uruguay (or people who have got a visible drug problem) take "pasta base" (https://en.wikipedia.org/wiki/Cocaine_paste; also called "paco" in some countries), whose effects are often compared to that of crack cocaine. That seems to be the drug of choice. Also beside the legal drugs weed, booze and nicotine in the last two decades there has been a steady increase in availability of MDMA (both pills and crystals) and LSD. Cocaine is easy to get and cheap. Shrooms and mescaline cacti are also easily available. But no opioids whatsoever (that I know of).