^For the same reason that Cartoon Network shows live-action movies and game shows, that Court TV began showing courtroom dramas and then turned into a reality-TV network, that former fine-arts channels like A&E and Bravo have turned into rerun/reality channels, that The Nashville Network turned into SpikeTV, and that countless other formerly niche cable channels have switched to more mainstream programming over the past decade or two. There is hardly anything new about this.
Commercial television networks need to make money to stay in business. You can make more money with programming that has broad appeal than you can with programming that targets only a niche audience. Therefore, non-premium cable networks that started out with niche programming have either diversified their programming or abandoned their niche focus altogether in order to stay in business. If anything, SciFi/Syfy has held onto its niche focus longer than a lot of other cable networks.
The only thing I don't understand is, if Syfy wanted to do a cooking show, why not hire Alton Brown to do it? Good Eats is practically a genre show already, full of food-science geekery, pop-culture references, cheesy MST3K-style special effects and skits, and occasional fantasy scenarios like visits from Santa and assorted historical figures.