It is because there no Su-27UBs available for entertainment flights in Russia…
… and in USA too.
http://www.prideaircraft.com/flanker.htm
Only single weary MiG-29UB still riding for tourists.
http://maps.yandex.ru/-/CVfCA0yv
Looks like only MiG-29 and L-39 available now.
http://www.bestrussiantour.com/jet_flights/prices_flight_mig29
In 2009 they sell MiG-31 flights as well.