Maybe because they've barely been training those pilots and know the planes would likely be lost if used above Ukraine? Pilots in the Russian Air Force have supposedly been averaging less than 100 flight hours a year, which is next to nothing.
That number supposedly comes from official Russian sources. I say supposedly because I don't understand Russian and have to trust western sources about this, but these numbers have been widely disseminated for years and seem to be broadly treated as credible.
Maybe because they've barely been training those pilots and know the planes would likely be lost if used above Ukraine? Pilots in the Russian Air Force have supposedly been averaging less than 100 flight hours a year, which is next to nothing.