My parents wont let me get anything new, why?

My mom told me a MONTH ago that we could re-decorate my room, so I started looking up stuff for me to redecorate with, and this morning I showed her everything I want and where I want it, and she basically said "No, you don't need any new stuff" I kept up my end of the deal and she dropped hers. I have all hand-me-down clothes and they're from my OLDER BROTHER, I, as a 16 year old girl cannot be wearing my 18 year old brother's clothes. I told her we could split the cost, and we're only looking at about 200 - 300 dollars. I told her I'd pay the rest and she still won't do it. We're not poor either, we live in a 2 acre - 2 story home and I go to private school, and I have a good college fund and we can pay for vacations all over the world, but she won't do this. Why is this the hill she wants do die on? How can I convince her that its a good idea to do some re-decoration? I am living in a 5th graders bedroom.