My understand of this is roughly:

* There's a fairly liquid market for sites of any sort that have a demonstrated ability to generate revenue.

* The valuation of those sites goes up when revenue is recurring.

* The valuation goes up with history/track record.

* That market is not especially oriented towards teachers.

Given the universe of different sites you could build that would charge a subscription fee to serve some niche of customers, it would be pretty weird to compete for the Bingo Card space.

(I was surprised at how liquid the market for this stuff was.)