I think the biggest reason is that state parties and candidates can tailor their platform in state races much more closely to the overall ideological make-up of the state than they can in national races. So Southern and Western Democratic candidates for Governor can actually be pretty conservative, while Northeaster Republican Govs can be much more liberal.
In a Senate election, the opponent can say, "Sure my opponent may SAY he's independent, but he still votes with his party 87% of the time" or whatever. Governors are not accountable to national party leader to anywhere near that extent...they actually CAN be independent. People are much more willing to vote for the opposite party for Governor b/c, for instance, voting for Mitt Romney for MA Gov. won't give Pres. Bush an extra vote in the Senate. For the same reason, voters are also more willing to oust incumbents; Governors have less of an ideological base they can depend on compared to Senators. Their elections are based on personality and actual performance.
In fact, in some states like MA, the governorship hardly has any power, since one party has 2/3rd majorities in both branches of the legislature.
