A full game’s-worth of development resources (time, money, people) can only support a certain amount of content. In this case it works out as four playable races.
If we were to add this extra content to the main game, we’d be operating at a loss, which we wouldn’t do.
So now we have a dedicated production budget to generate DLC in support of the main games, and we assign some of that cost to helping create a really great pre-order bonus. And what’s more we can make it available to buy from day one if you don’t want to pre-order. So the advantage is there if you do put your money down early, but you don’t miss out entirely if you decide not to.