Why pay an substantial premuim for a super light wheelset only to have it weighed down by a steel frame?
Most carbon wheels aren't that light; you can get down to 1200g for aluminium clinchers or 1000g for tubulars, so the advantage of carbon rims for weight weenies is very small even if that is the main selection criterion. However, there are no aluminium rimmed wheels which get close to Zipp 404s, let alone 808s, in terms of aerodynamics, and the gap is more than enough to make up for the 500g-1000g weight penalty of a steel frame. If you are racing on a limited budget, it's better to get the most aerodynamic wheels you can afford even if that means mounting them on a second hand 531 frame.
Most carbon wheels aren't that light; you can get down to 1200g for aluminium clinchers or 1000g for tubulars, so the advantage of carbon rims for weight weenies is very small even if that is the main selection criterion. However, there are no aluminium rimmed wheels which get close to Zipp 404s, let alone 808s, in terms of aerodynamics, and the gap is more than enough to make up for the 500g-1000g weight penalty of a steel frame. If you are racing on a limited budget, it's better to get the most aerodynamic wheels you can afford even if that means mounting them on a second hand 531 frame.