That's the main weakness of PvP gambling game especially if it is a lottery/raffle type game where player need to wait other player(s) to join the game. Most people usually want a quick game especially for an amount that can be considered as low amount (few cents to few bucks). I can understand that having PvP game only does not require the owner to have big amount of money because they are not losing anything except for the cost of the site and promotions. So maybe at the moment they do not have enough funds (as bankroll) to add regular games (players vs house).