Why did Russia sell Alaska to the USA?

Not surprisingly, the major reason that Russia sold Alaska
to the United States was the fact that the Russian Tsar needed
money.

Russia sold Alaska to the United States in 1867. 
The Tsar at the time was trying to modernize the country in a number of ways.  One thing
that was needed in order to modernize the country was money.  Russia felt that selling
Alaska was a good way to get some money, especially because they were not sure that they
could hold on to the territory anyway.  They had recently lost the Crimean War and were
somewhat afraid that they might lose Alaska in another war with the
British.

It is also said that Alaska no longer had much
usefulness to Russia.  The trade in sea otter fur was declining and there were very few
Russians actually in Alaska.

So it seemed like a good idea
to get rid of Alaska and Russia preferred to sell it to the US instead of selling it to
Great Britain since Britain was more of an enemy of Russia than the US
was.