If our company was hosted on Heroku instead of AWS, we would have to shut down. Want a 50gb redis? $4k/mo. MongoHQ's largest heroku instance is 150gb at $2700 / month? We'll need at least 50. Want 300 workers? That's $20k / mo for less quality servers.
Heroku is great for working on your own project or if your data scale is small. Otherwise, the cost is just too high.