I actually dig both Microsoft and Taco Bell, but putting that aside, WordPress is free in both its license and effectively in its price as well. Its biggest competitors according to W3Techs are also free, like Joomla and Drupal.

Given price isn’t a distinguishing factor and the other similarities among the top three, I would posit that quality is the primary driver of adoption and market share. It’s a free market expressing its preference. Popularity doesn’t mean something is bad, especially in economics of abundance (which the world is moving to).