Because in 15th century it was 6000 km from Grand Duchy of Moscow to Alaska. Same distance as for Columbus voyage, but not on the ships, just on your foot. All over Siberia. Without roads. In wilderness. And than you need to bulid a ship to cross Pacific Ocean. Well, it's just 82km at Bering Strait, but how do you know where to cross it?

Russia didn't exist yet in 1492. Its predecessor state, The Grand Duchy of Moscow, had all its territory in Europe and was in no position to discover Alaska.

Russians were very far away from North America during the 15th century.

Now, people living in modern day Russia, in the Chukchi Peninsula, had a more or less ongoing pre-Columbian relationship with parts of Alaska. It just didn't have much impact because the Chukchi itself was basically isolated from the Old World and the relevant parts of Alaska were almost entirely isolated from the New World.