Because a certain number of new coins are being created each day through mining so the supply of coins is increasing and if demand doesn't increase to match it the price is going to fall. It isn't a perfect calculation because those mining could hold the coins and not sell them immediately but the fact remains that the total supply is increasing and will at some point be put into circulation.
You will be hard pressed to find any serious miner who is keeping mined Doge instead of converting them to BTC or fiat. Serious mining farms pay for their electricity, thus they will not have amortized their GPUs solely from Doge.
Sure, if you are a gamer with "pre-paid" graphics card AND "free" electricity, you may elect to keep your mined Doge.