This question just occurred to me.
My first thought is that the closer your capital is to the rest of the world, the easier diplomacy is; we haven't always had telephones.
The flip side of this thought process, however, is that Moscow is responsible for a huge geographic area, and since it resides at the far edge of the landmass, communications to the rest of the country must have been, historically speaking, borderline impossible.
Militarily speaking, part of both Hitler and Napoleon's main objectives during their invasion were to get to Moscow quickly, occupy it, and the rest of USSR/Russia would fall.
So, why has Moscow been the capital of Russia? Specifically, are there other reasons than ease of communication with other countries?
The Grand Duke of Muscovy was the main Russian prince who ended up uniting the Russian principalities after the Mongol invasion. His power base was of course in the capital of his Grand Duchy, Moscow. After significant amounts of territorial consolidation, Ivan the Terrible was in a position to declare the Tsardom of Russia in line with that of the Byzantine/Roman Emperors, and retained Moscow as his capital until Peter the Great moved the capital to St. Petersburg.
tl;dr a convergence of historical circumstances and power shifts. Kiev was the original 'capital' and historic center of the early Russian state (as it coalesced from Slavic in-migrations) from around 800-1200. As ParkSungJun pointed out, the Mongols sacked many of the major Russian cities including Kiev, and at various points in time gave the Muscovite ruler the title of Grand Prince which helped it to accumulate power. Novgorod and Lithuania-Poland were Moscow's two main rivals - Ivan III conquered the former in 1478 while Lithuania-Poland acquired its own sphere of influence.
see further: Russia, Richard Pipes
One thing to remember is that while Russia has a huge land area, the land is not homogeneously developed. Siberia was only colonized in the 16th and 17th Centuries and the eastern stretch of the country are nowhere near as developed as the parts that are to the west of the Urals. So Moscow is in a reasonable place to administer the heartland of the country.
As another commenter highlighted Moscow was not the capital from 1712 to 1918, it was shifted to St Petersberg. St Petersberg was preferable to Moscow as it is a port on the Baltic, making it closer and more easily connected with the rest of Europe.