The reason is plain: The US and allied right-wing governments in Latin America don't believe they can win a war in Venezuela:
"Despite calls for military intervention, no major government involved -- U.S., Brazil or Colombia -- is planning such a move. Those who oppose the idea say it’d require tens of thousands of troops and billions of dollars in a country twice the size of Iraq with both a standing army and citizen militias."
Meanwhile, the US continues to strangle the Venezuelan economy in an effort to provoke a split in the army and a coup backed, if necessary, by invading forces dispatched by Bolsinaro in Brazil and Duque in Colombia.