How do I convince my Parents to get me a Hamster?

I am pleading for a hamster. I don't understand why I cannot have one. I have 3 dogs and they are german shepherd, american eskimo, and chihuahua. I take wonderful care of them. I walk them and give them exercise. I have 2 large fish that I feed and take care of. I have enough money to pay for all of the supplies. My parent will not give me a strait up answers. I wrote a report on them last year. I know everything about them. I know I'm sounding like a brat but really I don't demand a hamster I always say please and thank you. I ask them and I listen. I ask them the reason why but they just say "Because I said so" My parents do not say it rudely though. They laugh while I ask. Please HELP! Thank you for you help :)