Upvote:3
Russia expanded (mostly) eastwards for the same reason Britain expanded over seas: the path of least resistance.
On the West and South Russia faced powerful neighbors - Teutonic Order, Sweden, Poland, Austria, Ottoman Empire/Crimean Khanate. (Do I need to mention the Arctic Ocean on the North? ;-)
On the East there were rapidly decaying but still meddlesome remnants (Kazan, Turan, Astrakhan) of the Golden Horde. Obviously this is where one would apply the force: relatively easily secure the Eastern border and only then concentrate on the West and South.
Of course, Russia was, in fact, expanding in all available directions, wherever there was a weak spot in the neighborhood. It's just that there was much more space in the East and resistance there was much weaker.