If you have a Nexus, sure, you get any new Android versions the day it is released itself. But on the other hand, if you have devices from Samsung or other Original Equipment Manufacturers, then they will first have to skin that particular Android version before they release it for your device.
Skinning is often a tedious job, and making things creative in the new Android version will take some time. This is why we are getting 4.3 Jelly Bean in the first quarter of the coming year for M.
Since KitKat has done away with fragmentation, it has the potential to even run on older devices (devices released after mid to late 2012). The Xperia M is sure to get a KitKat update, but that will be a long time in coming. It's pretty obvious that we will get CyanogenMod or its derivatives or AOSP and its derivatives long before the official one.