why won't my parents let me sign up for the gym??

I would sign up by myself , but u have to be 18 or older or you have to go with a parent to sign up.. Anyways I've been wanting to go since the summer cause I am a bit over weight and I really wanna workout .. My dad promised me he would sign me up in the New Years and my mom didn't care but now he said he changed his mind for no reason and I'm so mad .. All my friends signed up and the gym is right across the road from me and it's an all girls gym and it's only 20$ a month.. We have the money and my sister goes there too .. My parents have absolutely no sympathy for me . I've tried crying infront of them many times and they just don't care and whenever I talk about it they yell at me :( I can't do anything outside cause it's winter and I hate at home workouts .. How do I get them to let me go ?? It feels like nothing will work and I'm sick and tired of this I just wanna workout like wtf how are they just gonna Change their mind after they swore they would let me .. When I ask why they say nothing . Please help :(