Your assumption of what the graph displays is wrong. Yes, it lacks a lot of information and the post could have clarified more.

But at the bottom of the graph you can see that the x-axis is years. Which is a strong indication that this graph displays the life expectancy of latin american countries. Whicha quick goolge seems to confirm. And it shows that El Salvador ranks poorly even amongst them. Since most migrants move to a country for a better life, the pool of countries that El Salvador can pull from is rather small.

But that obviously misses the point that many people who would move to El Salvador on using this opportunity either move there to help improve the situation for the average person. Or at the very least would have enough money to afford a better lfiestyle and not be affected by the average life expectancy. Which obviously is going to be low for a country that suffers from poverty and gang violence as El Salvador does.

So the "general" assumption of migrants moving to a "better" countries doesn't quite apply here.