I imagine it's because it provides a way to sock away $7k each year in a retirement fund that can be invested in index funds and won't get taxed after 65 on qua

I imagine it's because it provides a way to sock away $7k each year in a retirement fund that can be invested in index funds and won't get taxed after 65 on qualified medical expenses making it more valuable than either traditional or Roth IRAs that get taxed on withdrawal and when you put the money in respectively. High-income families have the spare cash to do that, so for them it's a valuable investment vehicle.