The statement from the Press Secretary is that the evidence against the Hammonds was weak. That the Hammonds are fathers and family men was also highlighted.
However, there is some degree of controversy over the Hammonds’ charges and sentencing. The Hammonds were charged under anti-terrorist laws which, to many observers, seemed a bit extreme for what was probably them trying to burn off brush from land where they wanted to graze their cattle. These terrorism charges really amped up the minimum sentencing of the Hammonds. The Citizen action in the takeover of the Mahleur HQ was ostensibly to highlight the apparent unfairness of charging ranchers with terrorism charges for a brush fair.
But while there might be some reasonable reasons why one might grant the Hammonds clemency because of an apparent unfairness in charges and sentencing, the President has instead chosen to highlight perceived weaknesses in the cases against them and their familial ties in granting them pardons. Whether that’s because bringing up the trumped-up (haha) terrorism charges would be poor optics, or that the administration was honest in its reasoning for the pardon, or if this is the result of a guy who is more interested in concerns about celebrity and fame than governance and the Hammonds are something of a celebrity case, I couldn’t say.