Why? Fogg has sucked his entire life. He sucked as a Pirate, he sucked as a Rockie, and he'll suck as a Red.
Belisle on the other hand showed real signs of breakout last season. If you dig deeper than ERA, you'll see a guy that demonstrated very good control last year, while missing enough bats to survive as a starter. A 2.91 K/BB is nothing to sneeze at, and really that's the telling stat with Belisle. There's a reason why basically every single projection system has Belisle posting an ERA below 5.00 as a starter this season. Add in the fact that he's younger and may actually have a future here, and you have a guy that is lightyears ahead of Fogg in being able to contribute to this organziation.