Why were the particular US attorneys fired? Ask Rove - under oath.

Here is an interesting analysis of the likely impact of putting new (and presumably more partisan) US attorneys into the districts that were targeted last December.

Short version -

6 of the attorneys come from states that are not completely red or blue. The other 2 are from the country’s largest state and a major source of campaign contributions California.

[Hugh gets credit for this, and more.]

This move was pure Rovian "manipulate the election system" tactics. Ignore what it does to the credibility of the Department of Justice or even more basically, to the "Rule of Law." This is just one more Rovian power grab action.

That's not to say it doesn't have other reasons also, but the manipulation of elections, especially the 2008 election, appears to be the primary reason.