I think we tend to make a bigger deal of the Monroe doctrine than it really was to people at the time. It wasn't an international agreement or anything, just a general statement of foreign policy - which the USA couldn't even enforce.
It's funny, I think we don't make enough of it. Given that it serve both British and American interests I think it goes a long way toward explaining forms of reticence in New World policy post-1823 that otherwise seem inexplicable.
I think it can be explained that there were easier conquests elsewhere in the world. France and the UK did flout the doctrine on a few occasions though.
France sent a bunch of warships to Haïti to collect reparations payments for slavery (which it continued to collect all century), while the UK continued to conquer northern North America, and also established a new colony in the Falkland Islands.
No one else really had a reason to go conquering again in the Americas. Spain was exhausted and in turmoil, while the Portuguese and Dutch were busy building their empires elsewhere. The lands of Africa, Asia, and the Pacific seemed like easier targets.
Importantly though, the USA (AFAIK) never did flout their own doctrine. Whilst I can see the UK getting away with it because they were required to enforce it - I'm not sure how the Haiti situation is flouting it.
Then again, it was routinely brought up - especially in Venezuela.
However, it wasn't completely disregarded by the main parties. If that happens, nobody else will pay attention to it. Heck, regardless of the Monroe doctrine - any of the above scenarios relegates the USA from a rising power to yet another political entity in the Americas, with Britain and Spain (and in scenario 1, France) able to assert control.