As others have said, the slant helps with stability. But the sneaky flip side to your question would be why aren’t all wheelchairs slanted? The main reason is probably because that weakens the axle. A straight axle will be stronger than a bent one with an angled connection. So to compensate, sports chairs need to be better engineering, making them more expensive. Since your average pensioner isn’t going to be doing too many doughnuts, the extra stability just isn’t needed.
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