From what I've gathered in the book, the answer is that Mariinsky Ballet management -- the eternally "interim director" Yuri Fateev and his right-hand persons managing the company and making casting decisions, such as Ms Gumba -- demands it. Pawlick is far too diplomatic to say those two names outright...but she quotes the various personal coaches of dancers and Academy teachers (not always the same people, although a few Academy teachers do coach the pros). Pawlick and others DO mention that the pro dancers' schedules are crammed, so that there is precious little time to work with personal coaches on nuances of roles. Pawlick quotes some coaches explaining how many soloists are forced to sneak-in sessions with coaches who are not those officially "assigned" to them by Fateev & Gumba. Unlike the old days (since either Vasiyev or Fateev, not sure who), dancers cannot request an specific personal coach; I know that they could stick to a favorite coach during the Vinogradov era.
The main thing to remember in all of this: the coaches of the pros & the Academy teachers are very often working against the tastes (Balanchine/Forsythe/"modernity" Uber Alles) of Fateev. Please do not equate the bulk of the Vaganova & theater coaches with Fateev and his tiny circle. Dancers' schedules are crammed not just because of the many ballets performed & tours but also -- just hinted at by Pawlick -- for the purposely-devious desire to kill the 'old' Vaganova elegance for once and for all.
Perhaps Fateev thinks that he can make more money selling the Big Mac, instead of a meticulously-prepared elegant meal? On the Good News Front (and this is too recent to have made Pawlick's tome): Perhaps the recent "winds of change" in casting are a small hint of a "correction" to Big Mac Aesthetics?