Why are the fashion designers and high end stores so intent on alienating a significant corner of the market?

It actually has gotten signficantly easier to find plus-size clothing for a range of styles, budgets and ages since the recession, for exactly the economic reasons you've pointed out. Is it perfect? Nope. Still have a long way to go and it is an underserved market still. Most options are still available mainly on the internet and through catalogs, though, not your local mall.