Because the iPhone 5c is a newer model by one year. It should get the same number of updates that the 5s would.
It's newer only by time of manufacture. Internally, it's old. You bought a brand new outdated phone, congrats. The 4S was manufactured at the same time as the 5, that doesn't mean it's going to get the same amount of software updates as the 5 because it has older internals. Apple doesn't judge by year of manufacture when deciding if a device is getting another update or not, they judge on whether the hardware can handle it. Hate to break it to you, but you own an iPhone 5 in a plastic shell.