You’d think there’d still be people who would do it when they are leaving office for good, if it’s just about politics.
I think many politicians believe the law is inherently good.
At least one ex-governor, although he didn’t advocate it until his second term, when he was no longer eligible for re-election.
There’s another aspect besides “Politicans in the US don’t want to be soft on crimes”: the advocates or criminals themselves may not want it.
When the justice group (a group of law students and a professor at some Northeastern University that research death penalty trials and often uncover obvious evidence of miscarriage and innocence) published several years back that about 100 sentences in one particular state were plainly wrong - that is, strong, undeniable evidence of not guilt - they wanted the governor to improve the justice system.
Instead, on the last day of his office before he went into retirement, he pardoned all the criminals, which meant that the flaws in the system that caused the wrong sentences in the first way were not done away with; and because the sentence itself was not lifted as a nullification would have, the innocent people were still listed as “guilty but pardoned” instead of “wrongly charged, really innocent”. This has effects later not only on the record, but also means that they can’t press charges against the state for miscarriage of justice by gross neglience or active conspiracy (as happened in some cases, when evidence was suppressed, ignored, mislaid etc.)