It's not unlike Google Search. Google Search has atrophied over the years but because it's still the best in the market, it's used by almost everyone. Competition is hard to build because it has to be better than Google Search in order to bother using it.

Heroku competitors have struggled in part because Heroku is a fully featured platform. It's relatively easy to build a platform that ticks a couple of boxes really well but building something that matches Heroku in feature parity is a daunting task. In order for competition to get there they need customers and funding, and funding is way easier to get the more customers come in through the door.

Once Heroku dies (perhaps already since this incident) we'll start to see real competition in this space because their competition will be getting used. The PaaS space needs that oxygen Heroku is taking up.