Evangelizing is tough, which is why most technology evangelists work for companies with enough money to buy small European nations and have vague job descriptions that they can't possibly fail at. I, on the other hand, have a limited budget and a well-defined brief.
The audience of biology teachers is much, much wider than the audience of "people looking for biology bingo", and it being more diffuse makes it harder to address via scalable marketing methods like SEO and AdWords. They're also much harder to convert, because I have to sell them on a) biology bingo, b) Bingo Card Creator, and b) paying $30 for it, whereas if I just try for folks who typed [biology bingo cards] into Google I get (a) for free.
I want to be someone's first bingo game about as much as Apple wants to make someone's first cell phone.
P.S. The game many Australians think of when they think of bingo is not the game on my website. Yay, internationalization.