White bread lasts longer than whole-wheat bread, and white flour is the cheapest available flour (well, since industrialization. Historically it was reserved for special occasions and the social elites) . Also Wonderbread is available in whole wheat. Also also:

In the United States, consumers sometimes refer to white bread as “sandwich bread” or “sandwich loaf”. [9] It is often perceived as an unhealthy, bland, and unsophisticated menu item.

Hostess, the makers of such delightful American™ staple foods as Twinkies, Ho-Hos and (for many years) Wonderbread, went bankrupt more than a decade ago, so like… they weren’t (and aren’t) a huge part of the american diet.