I was recently in El Salvador, which uses the USD as an official currency (alongside BTC lol). Despite using dollars, the $1 coins are used instead of notes almost exclusively for that denomination (mostly presidential dollars and some silver Susan B Anthony dollars). I was curious and did a bit of research, apparently the reason is that because day-to-day transactions are done almost entirely in small amounts of cash the paper notes have a very short lifespan (apparently <1yr) there, while coins will last decades.
Yeah, that makes sense. Coins are clearly more durable, and for denominations that are handled frequently in places without access to federal reserve replacement, durability wins over preference / aggregate weight and volume.
In mainland US, preference for paper $1s isn't unduly burdensome. As a merchant, deposit the worst of the $1s and use the rest as change, no big deal. As a consumer, if someone doesn't take your grimy, half torn $1, try it a couple more places and then take it to a bank (perhaps even your bank). But it's unsurprising that doesn't really work for El Salvador.
