You can play the game without Steam. Don't see why you're not able to play it without having to use Steam.
Because a lot of people like me don't use credit card or paypal but alternative forms of payment which steam provides them.
Actually it's not, especially for small indie game developers like HTC.
Unpopular games on steam on average have 275,602 copies sold. The lowest with 5,658 and the highest with 1,201,136
you can see it all on
www.steamspy.com.
Warbirds which is pretty much dead and you could call it very unpopular have 4,756 copies sold and stats shows that in the last 2 weeks it had 1700 players ! (probably playing single player as multiplayer there is pretty much dead).
Just go and input 'warbirds' in steamspy page if you don't believe me.