Because it wasn't sold with a bundled Rekordbox licence. The mixers that do unlock were bundled with a licence.
I'm not sure why Pioneer are doing this but I've two theories:
1. Simplicity
2. Most low end Pioneer gear ships with a licence. Pioneer wants anyone that graduates to use their high end club gear with Performance mode to have to pay for the privilege. Down the road if streaming gets added to RB then people will want to use that in clubs and will have to pay to do it via subscriptions.