How do I convince my parents to let me adopt a cat?

I'm sorry this is long, I'm just trying to provide enough information!

Well I'm 14 and in grade nine. I have two dogs and my cat was recently put down, about 2 months ago. I loved her so much. I have grieved and now would like to add another feline to the family. I really want to adopt a kitten from a shelter. No matter what I do, my parents say no! Here's a little more about myself :

I have a 85% + average at school, honours student.

I am an avid volunteer for the Toronto Cat Rescue, Paws n Tails cat rescue, and I feed feral colonies in my town with a team of cat rescuers.

I stay out of trouble.

I am responsible with my dogs, I feed them, I walk them, and I clean up their business in the yard.

I do what I am told and I don't understand why I can't have a kitten. It's a lot of responsibility and I fully understand that, I am willing to give up my time and energy to take are of the kitten. My mom is slightly allergic to cats, though she never had a reaction to my old cat. I promised to vacuum often and brush my cat daily to keep shedding down to a minimum. I have had other pets and have kept my promises to care for them.I have had a hamster, a guinea pig and I recently lost an amazing budgie to heart problems. Money isn't an issue, the shelter I'm looking to adopt from (paws n tails) has three litters of kittens with their first shots for $140. I babysit a lot and save all of my money (i have over 3 thousand saved..) so I offered to pay for everything necessary. I only asked for financial help with some vet bills if necessary. I will be taking the cat with me when I move out. I really don't understand the issue! Can somebody please help me convince my parents to let me adopt a kitten?

By the way, I have an older sibling in university and a younger sibling who doesn't want anything to do with a cat. Which is purrfect because I want my kitty to myself. Though I'm willing to share if need be. Also, I would prefer a kitten so I can leash train her (I have a lot of patience, I've leash trained before) and mould her more into the cat I want. (Though cats tend to do their own thing no matter what! Haha) I am also willing to adopt an adult cat. They are just as beautiful as a kitten and I am one of the few kids my age that understand the over flowing adult population in shelters. Any techniques I can try? I am really serious about getting a cat and have wanted another for years, but my old cat didnt get along with other cats. I don't want to beg because it never works. Thanks for any help! Again.. I'm SOOO sorry it was so long!