Why rewards in splinterlands reduce ?

I have played splinterlands for 2 month. In first time I received more good rewards such as epic card, rare card, sometime gold card and more DEC when open daily quest about 2000 DEC each open. It is time I very focus to play game. I earn more each day when I optimize rent card. I rent gold card pro card, alpha card. I often play water type in bronze 1. Each battle I earn about 7-8 DEC. Near time when amount players were increase, my rewards is reduced. Everybody like play splinterlands so each battle is high competion. This is main reason make rewards in game reduce.