I agree about down-voting due to gaming, but I don't see the reason on up-voting. Can any one game the system with up-voting?
The main reason for upvotes being locked is (probably) that it was the easiest way to implement it server-side.
However, the reason that I can think of is that User A (Billy) might have an answer to one of those SQL 2013 questions that scored Billy 40K rep. If, when SQL 2014 server comes along, and every user, Aaron to Zzap, removes their upvotes, Billy will come here to Meta and ask something like "what in the world just happened to all of my privileges? And my reputation?
Now imagine the problem multiplied by every other new SO user who gets their newly gained 50 rep taken away just because the question they answered isn't current anymore. Imagine how many duplicates there would be - the mods would resign after about 3 months of this 'feature' being implemented.