Tokenism is a valid answer.
Governors who win primaries are more likely to be white guys, and they're more likely to pick someone who can add diversity to the ticket for the most ceremonial post of Lieutenant Governor. The post isn't very prestigious, and the pool of potential Lieutenant Governors is big enough that potential Governors can find a woman or racial minority as qualified as any white guy who would take the job. There's usually a state legislator, failed congressional candidate, prosecutor, academic or state party chairman available for the office.
It really depends on what the role of the Lt. Governor is for each state too. In some states, such as Ohio, the Lt. Governor runs a department, so she's basically an elected cabinet member. In others the Lt. Governor does nothing at all.
