You just proved my point.
1. Business strategies. They are trying to see how a character with no large install base (like Skarlet and unlike Rain and Kenshi) and less fan demand will sell, so that they can an idea of how largely requested characters like Rain will sell. It's like with box office, you can't judge how much Super 8 would have grossed based on Transformers 3's box office, as Transformers 3 is an established franchise with a large built-in fanbase, like how Rain is an established character with a large built-in base. Skarlet corresponds to Super 8; Super 8 is an original property with no built-in fanbase, and Skarlet is a new character with no install base to anchor its revenue. Why do you think Skarlet was before Kenshi, and Kenshi is before Rain? Rain would sell well regardless if he was the 1st, 2nd, 3rd, 4th or even the very last DLC character, so he's not a good gauge at all for how their DLC characters will sell.
2. They are trying to keep people supporting the game and attention away from their competitors. Releasing their big guns right away would ruin that, and it would also eliminate much of the demand for future DLC characters (like Skarlet and Kenshi).
I don't really agree with #1 because everyone I know is collecting all DLC, either it's Rain or someone else. But you are totaly right with #2. I didn't thought of it that way. Keeping the hype about Rain will keep the fans interested in MK.