Why won't my mom let me get a pet rat?

I'm a sixteen year old girl. I have two dogs, and I've had guinea pigs for a while now, but my last one just died ='(. When I first got him (as a friend for one who's friend had died), my mom said he would be my last small furry pet for a while. I agreed, but I just lost my two piggies within two months of each other, both from untreatable conditions, and I REALLY, REALLY miss them ='(. I've always taken great care of them. Our vet even said they had a job waiting for me there if I ever wanted it because she noticed my special love for animals, even though I'm not old enough to work there yet and I hadn't asked about a job. Anyway, right now I'm pet sitting for a friend who has rats, and they seem so friendly and happy and cute and playful and fun and...I could go on for hours. I love the way they climb around like little monkeys and groom my fingers when I stick them through the bars of the cage. Anyway, I really want one, but my mom won't even tell me why I can't get another pet. She said even if I wanted another guinea pig, she wouldn't let me--and believe me, she loved my old piggies. So why won't she let me get another pet? She refuses to even explain why I can't have another. All she says is that we're just done with them, and I don't understand.