Mexico is in a tight spot. They are facing a lot of heat from the US, for not stopping illegal immigrants. But, they are heavily dependent on the US for trade. So, they are not going to go against US decessions strongly. So, Mexico technically supports the US decision.
But If you look at Northern Hemisphere, Chile is Burning. Brazil is struggling. Everyone knows about Argentina (which has been in distress since 2 decades?). Columbia has severe issues. All countries (Big ones) in the southern part of the hemisphere are struggling in one form or other.
There are millions of people in these countries and had been struggling. They have been suffering and expecting a stable environment to survive since mid-2015's or so. Once people come to a belief that, the situation isn't going to change any time, they are left with only one option. To migrate elsewhere. Then the mass Migration starts. But, where can they go? Only the US is the stable nation in the northern hemisphere, to which they need to go. (Canada exists, but they need to pass through the US or cross Antarctica). To reach US, they need to cross Mexico. But Crossing Mexico, to enter the US is difficult now. What happens then? Majority of these migrants will end up in Mexico. Life is already difficult in Mexico and people are finding ways to sneak into the US. What happens if the flow of migrants rises? what happens during the transition?
Russia and others support Maduro, because, they have vested interests in the supply of resources from Venezuela. Also, they need access to ports, to attack their enemies, if needed. Canada slides with the US. So is Mexico. They need a base if the war starts. They need access to ports, etc. So, they support the remaining nations, for access, if needed. Its an investment for them.
For Mexico, its do or die Situation. If they condemn Maduro, it will enrage its citizens to fight against Maduro. This could initiate migration, which they do not want. If they slide with the US and if the situation worsens, it could again trigger migration. The only way they could solve any potential upcoming issues, is by being gentle and encouraging the ruling party to be considerate to its citizens, and by encouraging Maduro to be a good leader. (Coaxing?). This way, they can delay or (possibly) aid in improvising situation in Venezuela, for the better good.
The US is not in a position to take any action on Venezuela. They have their own issues and are working hard to fix them (Trade Wars, falling jobs creation rate, inverted yield curve, Rising Poverty, etc). Acting on Venezuela now will be an expensive operation, which will drain a lot of US resources, which they don't intend to spend. So, they are just staying calm for now. They do not want to trigger anything. But they strongly oppose Maduro.
The US doesn't want to act. But the US opposes Maduro strongly. What options does Mexico have in such a situation? They just wanna Slide with Maduro, so nothing big gets triggered.
There could be other possible reasons (Bribery?) which aren't public. Also, I could have missed something too. But in my opinion, this is the primary reason.