Well, the obvious has already been stated Sales, sales, sales.
I also wonder...did Susan make good contractual deals with the Q, saying how often she'd be on air, have TSV's etc. I'm pretty sure that stuff is negotiated and part of their business deal with each other. And she continues to perform $$$ wise so, yes, that's how.
Also, I've often thought SG cornered the "Chicos" thing but on Q...right when that stretch/nylon, travels well, "easy wear/easy care" knit became a "thing" and so popular for all those reasons above.
My mom loved her things (as well as D&C, Louis, and other brands no longer on Q.) SG tops were flattering on her, and easy as she got older and preferred comfortable, stretch items...with flattering styles, colors & shapes. (Mom was a Chico's fan too, for the same type items.)
I've owned some of Susan's items too, and liked them. Good quality, design details.
Last...Susan is an old school fashion merchant who knows the design terms, still shows up on air to talk (like her style or not) about the design and mfg details. She's truly been passionate and a workhorse, about her brand, for years. She deserves her success.