Europeans were already interfering in Latin America, with or without the American doctrine. To be honest, the biggest protector of countries against European influence was England, not America. And even so, more than once England almost went to war with American countries, as happened with Brazil. With Brazil winning the dispute diplomatically. As a whole if we are being honest the monroe doctrine is more of a make believe than something real in this era. As a whole, the US interfered in the Caribbean, the region they managed to interfere with because they didn't have a strong enough local opponent.Of course, with a weaker USA, Latin America have to deal with another major destabilizing factor; the Europeans.
Slavery was a reason but it was not the main one. Brazil failed for a wide range of reasons. Being colonized by Portugal being one of the most important, the country did not have the mass of people or power to win important disputes against Spain. So Brazil was unable to secure the Plata basin region (it would be the same as the USA not having the Mississippi basin). However, unlike the Mississippi River, the most suitable area for Europeans in South America is the Plata basin region (and the most fertile too).The big reason Brazil lags behind the US is because Brazil was a slave empire, while the United States was mostly free.
The same country did not have a culture of appreciation for education. To give you an idea in 1800, Portugal had two universities (France had a total of 15 universities, he United Kingdom had a total of six universities and Spain had a total of five universities). And It prohibits the creation of universities and teaching centers in the colony.
Then, when it gained independence, the country had to basically go bankrupt to pay for Portugal's independence. This payment is caused by interference from the UK, which intervened to protect Portuga against brazil. When the country was reforming and beginning to advance, modernizing under the reign of Dom Pedro II, a coup d'état occurred by positivist military personnel in conjunction with the slave elite for the abolition of slavery. And instead of destroying the revolt, the emperor, who was in tremendous depression, preferred to abdicate. After that, these two groups destroyed the economy in less than 10 years. After that, the country staggered until today. Some moments going well in the economy and some bad moments, but being unable to really reform