And also put into consideration that more people are joining the blackjack casino, so there is every chance that reward would decrease over times since the number of users being rewarded from that pool have increased drastically.
Actually the total player has no effect on the prize because it has been set only the top 4 will get a prize so I think this is not the right reason
Another thing to note is why new players come to Blackjack.fun obviously they have a reason well if what they expect is not available then they can just leave.