Maybe it is because they are just a moderate priced brand of boot that ordinary people like me can buy for work, hiking etc-instead of her wearing a $2000 pair of faux work boots that look fancy-but are poorly made and impractical for actual outdoor wear-how dare she wear the shoes of a commoner...
You are probably right, however, her critics will find something “wrong” regardless of what she wears or does. If she’d worn the expensive designer boots, we would have been informed how long it would have taken the military folks to earn enough money to buy those boots.
I still see remarks about her “inappropriate” footwear when visiting Houston after the hurricane. IIRC, she wore heels from the WH to the helicopter but was in tennis shoes when on the ground in Houston.