It does not necessarily mean that popular restaurants always use white vinegar and only high-end restaurants use akazu red vinegar. Some first-rate restaurants do not use red vinegar at all. However, a new menu with red vinegar was introduced by Sushiro in 2018, even though customers can dine at the popular chain restaurant of kaiten-zushi (conveyor belt sushi) under a budget of 1,000 to 2,000JPY. Each restaurant has its own recipe for sushi vinegar: the chef may cook rice with kelp broth, mix white and red vinegar, or more than two kinds of vinegar, to season rice, or use them separately in accordance with what kind of fish you use.
Some people say that akazu red vinegar has been rediscovered because of the current popularity of classic, 150-year-old, Edomae style sushi, such as Kohada (Gizzard shad), Anago (Conger eel), and Akagai (Ark shell), rather than creative pieces that use caviar or truffle. I agree that this can be a reason, but I would rather think that constant research at high-end restaurants led to the use of various vinegars and consequently more restaurants have started to incorporate red vinegar into their recipe (In other words, it is not that these restaurants use only red vinegar; rather, they create their original vinegar by using red vinegar as one ingredient).
Recently, chefs use not only white vinegar to season rice. They cook rice with kelp and other ingredients for flavor or mix more than two kinds of vinegar at a ratio different for each restaurant. Sushi rice alone requires the careful adjustment of sweet and sour flavors.