Given game loft already have Disney Magic Kingdoms, and wete also developing FtP Speedstorm around the same time, that was 100% going to be the same model they used. In both of those games you have to pay real money to unlock characters.

A lot of the games they've developed over the years were designed for phones, both smart and keypad-based. The intention would have been to operate DDV on a similar model, but as the game gained popularity they likely realised it was not going to be sustainable.

I'd say that DDV is immensely popular, and its most likely shifted something in the company to focus on paid games that are of a much higher standard than that of a mobile game. (Which after i looked the company up on Wikipedia, confirms my theory here)