Mexico City is one of the largest cities in the world. Five people out of 21 million is nothing. Sure, every death is tragic, but there are a lot more efficient ways to spend time and money preventing deaths than tree trimming.
Five deaths in the Bay Area from one storm, two in San Francisco (with a population around 800,000). There were other tree related deaths last year. More to the point, the trees that the city is actively removing are old, mature ficus trees that present a variety of risks (e.g. sidewalks, sewer and other underground utilities), overhead (e.g. public transit) wires.