"Why Riot doesn't limit Smite to 1 per team?"
Because then people could troll during champion select, by selecting
then prohibiting other people from selecting it. This could be a problem during blind pick because even if another player called jungle first, you could just select Smite, and not de-select it, you would be guaranteed to go jungle. During draft pick, it would be harder to steal other's lane, but it would be a lot more impactful if it was a ranked match. If you were quick enough you could just steal your teammate's lane, and even if they were to choose to go jungle because they were above you, they would be forced to go into another lane. This would cause countless affrays and would foment toxicity.
I am saying that because some people are proposing that as a means of solving the problem of laners abusing jungle items that Riot introduce. Not that I agree with that, but if you want to solve a potential problem of laners abusing those jungle items, there are better solutions, this one could cause a lot of problems due to the above reasons. To limit Smite to 1 per team and still avoid that problem, Riot would have to rework the entire jungle system and that would be a lot of work and time spent into that.
this item has an effect that is very powerful, and even laners want it, it is so powerful that laners are willing to give up a summoner spell that would be a lot more useful to them otherwise, just to get it. Challenging Smite is powerful because it is essentially an Exhaust on a low cooldown.
enchant never had this issue because it doesn't offer anything unique that can't be aquired any other way.

by making it so that his Q automatically turns his next AA into pure magic damage.
even without the super unique passive because of the combination of stats, but honestly if they changed Lich Bane so that it converted base attack damage to magic like runeglaive does, then this wouldn't be as much of an issue.
and there, mana problems solved.
I know some people take it for the waveclear. But really, that isn't worth giving up a whole summoner spell for.