The reason for this is pretty straightforward and rational – the current state of affairs suggests that the source code of the final Android L release will be much different from the one in the already-available Android L preview. While said changes could be either minor or significant, the developer does not intend to waste precious time and try to implement the mouth-watering features of the upcoming Android update in CyanogenMod just yet.
We can't fight with the sound logic behind this reasoning, but in the end, it comes to you, regular Android users! Would you rather wait for a stable CM 12 build or demand the CyanogenMod-flavoured Android L goodies as soon as possible, even with an abundant set of bugs and flaws on board?
source: CyanogenMod via Android Authority