Because games that require for you to wait an extended amount of time to actually play and stop you from playing are annoying. Sure it shows your devotion if you keep playing, but that's dumb. I don't see why you would play a game that stops you from playing after an hour or two. How is that fun?
I get fun from quests because they give me challenges and I can complete them without interruption. If there's ever a time limit to how much you can play then they're not worth it.
If you ever have to start paying for quests then that's when it starts going downhill. With quests they want to encourage people to play more games (it's working), but if they add quests to games that require for you to wait/stop you from playing or add quests that need to be paid for, expect to lose players and get negative feedback. It'll of course give ArmorGames more money and support them, but there are other, less annoying ways to do this: Premium content (may make quests easier, but not required to achieve), premium membership (don't see ads and get special offers), etc.