Why not? What does Hasbro have to lose? Potentially damaging a brand in an extremely small market segment that honestly doesn't bring in that much money? And what do they have to gain? If they happen to hit it reasonably big, they get a slice of a box office pie that for a single movie would dwarf the entire tabletop industry, plus a perfect opening for making even more off of associated toys. It's the equivalent of buying a cheap lottery ticket for them. If they run the numbers and decide that the (chance of hitting it big) * (projected revenue gain) is greater than (chance of damaging brand) * (reduction in value of damaged brand) then it could easily be a bet they are willing to take. And the projected revenue gain is probably a much larger number than the potential reduction in brand value, meaning that the chance of pulling off doesn't even need to be particularly good for it to be worth trying.
As a gamer and someone who likes D&D, I'd like to think that the D&D game is really worth something, but to the execs Hasbro it might just be an asset on the balance sheet, nothing more and nothing less.