A big factor, as I have mentioned, is the driver support for the new OS version. From what I have seen from Samsung, they have skipped 4.2.2 for devices that will eventually get 4.4, and have or are releasing 4.3 for now for devices that WILL get 4.4 at some point. The S3 is a 2012 phone, so will get whatever version of Android comes out in 2013 at some point(carriers may delay it by months). The S2 on the other hand is a 2011 phone, so will get 4.2.2 and that's it, end of updates. Basically, if you buy a FLAGSHIP, you should expect to see updates during a 2-year contract is my thinking.
The S3 itself did come in 1GB and 2GB versions, so we may see the update for the 2GB version rather than 1GB version as a POSSIBILITY, but 4.4 may very well make better use of resources, so may run just fine on the 1GB version. Basically, can the phone HANDLE the update without problems is a key aspect, and any device maker has to be really careful not to reduce functionality of a device just to release an update. Will KitKat be an improvement for phones with only 512MB of RAM? If yes, they the device maker MAY put in the effort to put 4.4 on even older devices, while a slow-down might be delayed until the update can be stripped down to work well in an older or mid range device, and THAT extra effort may make the device maker say "no" to that update.
So, I again say that the S3 should get 4.4, but that will be the end. Again, this goes on the track record of Samsung having released Jelly Bean to the Galaxy S2 and other devices that many would have expected would not get the update.
Another thought on 4.1, 4.2, and 4.3 is that you also have the whole, "What does this update actually add?" question. 4.2.2 vs. 4.1.2 on a TouchWiz device without a custom firmware(I hate people using ROM for firmware since it isn't accurate) adds WHAT? 4.3 at the least adds Bluetooth LE, which opens up the door for a LOT of Bluetooth devices that require Bluetooth LE, including the Polar heart rate monitors which currently are not supported on Android due to the battery running out quickly without the LE stuff.
Samsung and the other device makers were probably told NOT to say ANYTHING about 4.4/KitKat until the official release of 4.4 at this point, and it is entirely possible the device makers have 4.4 already and are working on firmware for their devices so they will have them ready sooner, rather than later.