The late Lieutenant-General Günther Rall (275 kills) sums it up here, starting at 01:00 minutes in:
https://www.youtube.com/watch?v=STFdRrWBW2w
The main factor seems to have been the availability of targets, I have been told the same thing by other veterans in conversations in which this subject came up. Another reason, not mentioned by Rall in this video (possibly so as not to ruffle any feathers?) was that in the first years the opponents on the Russian Front were very poorly trained and these unfortunates primarily ended up as the proverbial fish in a barrel. Later the Russians caught up and were on an even footing, in some cases even surpassing the capabilities of the Luftwaffe as the experienced old hands were killed and the average skill-level dropped. Rall, Krupinski, Hartmann, etc. all amassed the bulk of their tallies on the Eastern Front. Aces that did not serve on the Eastern Front such as Adolf Galland or Josef 'Pips' Priller did not achieve similarly high numbers of kills, although their records are impressive in their own right. Again, availability of targets and duration of combat-duty.
As an aside: a kill had to be confirmed by a witness, which meant that kills were sometimes not accredited. If two or more pilots made a claim they were awarded a partial kill only, and that, again, only if witnessed and confirmed.
A sombre yet fascinating subject.