Because Venezuela is a Russian ally.
He knew it the day he passed the Executive Order banning the Venezuelan cryptocurrency that he had, inadvertently, stepped on Putin's toes. The Cryptocurrency called "Petro" was not only State Backed, but was also a half-hidden joint venture between Venezuelan and Russian officials and Businessmen( Time reports March 2018) .
The "Petro" was meant to link the value of the Venezuelan Oil Reserves and pictures of the inauguration ceremony was attended by Maduro's closest Russian Advisors , Denis Druzhkov and Fyodor Bogorodsky, billionaires with close ties to the Kremlin.
VENEZUELA IS A RUSSIAN STRONGHOLD.
For now, Maduro, with the help of his allies, had withstood the impact of the challenge of Guaido to overthrow his Presidency. However, he is at war with his own people. With huge oil reserves, suffering hyperinflation, shortages of food, water, medicines, the situation is precarious and may not be continue without further bloodshed. The fruadulent elections of last year, have been denounced by most of the world and Maduro is anything but a legitimate President.
Given Russia's interests, for Trump to initiate military action , would antagonize his hero, and probably , that would have been the nature of the 90-minute call between the two friends yesterday. As he has tried and successfully done in Syria, Putin will make America useless in Venezuela. There seems to be talks between Trump and Putin to let Maduro escape to Cuba as per Financial Times reporting. Will Putin snub the US again or Will Trump play a sanctions game with Russia ? China has a powerful role to play here as well. A little intervention on their part could really change the entire power play. How far will this happen without violence? This is for us to see in the next few days.