Voted for Others!.

The second option of the poll ("They are cheap because they are given freely!") is not true, because in order to play Steem Monsters (and receive reward cards), you have to buy the Steem Monsters starter set (currently the Beta starter set), which currently costs $10 USD (or the equivalent in cryptocurrency), so the cards are not given freely.

The third option of the poll (They aren't cheap at all!") is also not true in general, because the starter set costs $10 USD, but most of the cards are currently worth only a few cents. My Steem Monsters card pack is currently worth only $5.01 USD.

The supply of the cards is good in general, but not good compared to the number of the players, so the main reason for the cheapness of the cards can be the relative small number of players. Not many (only a few hundred) people are playing Steem Monsters nowadays. Probably this is why Steem Monsters is Recruiting Sponsors and Advertisers.