It is odd how so many people want to run for this seat, but like no one wants to run against Masto.
It makes sense with some foresight - running against CCM would be an uphill battle that, if successful, immediately positions you for what would in all likelihood be an even tougher reelection fight in a Presidential year after six years of taking tough votes.
Running against Sisolak is probably a marginally less challenging campaign and positions you for reelection in what will likely be another Dem midterm, along with the position of governor by nature offering you more freedom and independence in terms of building your record.