That's the whole point. Many first day DLC are free for pre-orders, and this is exactly the same. They've just been marketing it as if it's extra content they're creating cause they're good guys. From my experience, you can't just cram in another character out of nowhere without it taking a lot of extra time. There's balancing, testing, game design, etc. that all take up a lot of time, especially if you suddenly have to do it again later on. This was planned from the get go.