I used to spend a lot of time in Heroic because often I'd find groups of people who just can't seem to complete the often tedious objectives (protecting the Garage and Temple or defending Generators and Forerunner Relics) in Mythic. But really, I spend much more time in Mythic and Legendary now because my friend prefers to run them whenever Mythic is up. All you really need is a Spartan buddy to go with you that is pretty good at the game.
But arguably, I think Heroic is where most of the population usually stays because it's a quick and easy way to rack up both experience and req points which is perfectly fine if that's all you really want is just an easy, fast, and rewarding game! :) Mythic and Legendary can sometimes feel overwhelming, so that's probably one of the reasons most stick to Heroic.