Mekansm VS Crimson Guard

Like many things in DOTA, it's very situational and more often than not decided on who you are laning against.

If you are playing melee and going against ranged in lane, a shield is pretty much essential. This naturally progresses into a vanguard and CG.

If you are playing razor / viper who are more able to stay out of ranged right clicks, then mek is more likely going to be beneficial.

You also need to look at other very useful Mek carriers on the team and decide if they are better off carrying a mek and you building CG

You also need to bear in mind that to get the +2 armor aura you need to activate the item. Crimson Guard allows you to do that prior to a team fight for 0 mana but Mek doesn't (because then you don't have the heal mid-team fight)

There may be a logical reasoning for your question, however, these are still situational.

The reason why Razor and Viper gets mekansm is because these are the types of heroes who can easily farm meka at the early stage which is very advantageous for team fight. Vanguard in the early stage is not hat effective for these heroes because all you need to do is orb walk (run and attack). These heroes does not require too much mana for other skills and if they do, they can supplement it with aquila which is an agi type ring. These heroes need survivabilty and I think, mekansm is best for them.

Next thing is why centaur is good with mekansm and not vangurad. Since the time that dagger was buffed (no mana cost), centaur only has 2 skills with mana. Meaning, he still has spare for meka. Also, centaur does not get easily focused in team matches (because he is very tanky), so vanguard is no use for him. But, for now, I rarely see cent with meka because there are other heroes on the team who can get meka.

Last is that, why axe should be vanguard and not meka. The main reason why you have an axe on your team with vanguard is because you want somebody to tank for your team. You might wonder, "so what if axe has vangaurd, we can just kite him and run around". The advantage of being an axe is on his "Berserkers Call". You force your enemies to focus on attacking you. This alone explains why you need more life and damage reduction. Also, in my opinion, axe should not get meka. Why? Do you happened to experience on a situation when an enemy axe jumps on your team mates and you have the sense on attacking the axe also to burst them? Yes, it's also with the mind games. Axe may not have the opportunity to activate the mekansm. This situation also applies to centaur, that's why I also centaur rarely gets meka.