why is it so difficult to fly to Moscow?
Due to lack of the Open Skies. Russia has not been eager to sign any agreement with the EU, so only selected carriers can explore this market, Ryanair can't. Unless they choose to try St. Petersburg West (EFLP) of course. Yet, it didn't warm them up.
EFLP is Lappeenranta in eastern Finland, the closest EU airport to St. Petersburg, two hours bus drive from the center of this famous city. It could lure thousands of Russians and serve as a gate to the West for them as well as give EU citizens access to Northern Russia. It would require however more than just flights to EFLP, e.g. frequent shuttle busses between St. Petersburg and Lappeenranta. Too difficult? Some years ago there were some short negotiations between Finnish Aviation authorities and FR, but Ryanair has been very sceptical about Russia's potential at the time. A success in Poland and other Eastern European countries could influence their stands, however. I was very surprised after being able to see tens of Russian cars at Tampere Airport. Many Russians drive there from St. Petersburg, some 460 kilometers(!), to be able to continue with Ryanair to London or Germany. Driving to Lappeenranta only they'd have 60% less distance to make. If MOL wanted to fill his planes somewhat better...