This is because Elly has an innate and exaggerated dread of physical injury. This means that she cannot enjoy sports because she pictures herself as the person who's bound to be horribly injured and left to rot in some hospital somewhere forever and ever or as the person who's supposed to give up everything forever and nurse a hopeless cripple and never be allowed to object.
When we factor in the fact that Elly is incapable of understanding that people have tastes that differ from her own, it becomes sadly obvious that this is why she fears and hates anything fun and enjoyable and can't open her tiny mind. She can't sit back and say that skating or skiing or snowmobiling or the like are good things she can't partake in; she has to make sure that no one can enjoy things she can't so she doesn't have to hear the cruel and evil word "killjoy." It's why she hates John's cars as well. They're too flashy and fun for someone who won't let herself enjoy life.