What alloy would that be? Aluminum? Doesn't have the temperature tolerance. Titanium? Nightmarish to manufacture and expensive. Composites? Temperature intolerance and they tried it; not robust.
Stainless steel lets the structure get very hot on entry without failure, beyond the temperature where aluminum alloys would lose substantial strength. This reduces heat shielding requirements.
The sky is the limit really. The shape and design is sound so now you can test all sorts of options. Internal frames to buttress the structure. Mixed materials. Composites. You name it. Steel is super easy and cheap for prototypes but if you know you can get the ship back you can play with all sorts of variables.
Stainless steel lets the structure get very hot on entry without failure, beyond the temperature where aluminum alloys would lose substantial strength. This reduces heat shielding requirements.