We do that on the lottery because people might want to check against the ticket list which they can still find the link to on our Twitter feed. I can't really think why anyone would want to check further back than the last wagering competition.
Well, it should be quite obvious. We want to calculate how many times people have won any of the previous contests, like we do with the Lottery. Any site like to gather statistics as proof that they had previous contests and many winners, because people will feel more comfortable if they see previous statistics of other people participating in these contests.
I have visited many other small gambling sites and I have checked their lotteries and contests and most of them are only won by 1 to 5 different users, which will raise a immediate red flag for me.