Well, there's this and the fact that browser implementers all need to come to consensus over what features to add and that's an extremely difficult process to move forward. There also is one big browser implementer who doesn't really want the "playful web" to exist and would prefer that everyone live inside of apps.
Edit: To be clear, we all work together and mostly get along. It's a long and arduous process to reach consensus and not everyone's incentives are aligned and this can be frustrating.
Edit: To be clear, we all work together and mostly get along. It's a long and arduous process to reach consensus and not everyone's incentives are aligned and this can be frustrating.