Says who? I'd say let's empanel a grand jury in deep red Texas and find out.
After all, if crimes can be strung together to indict one guy, why not another?

Says reality.

The reason that Trump was able to be indicted in New York is because he did something that violated the law. He created (or caused to be created) several business records that contained false information. Regardless of whether or not you think he should have been prosecuted for that, or whether he should have been prosecuted for the crimes he was charged with, those underlying acts were never disputed.

There's nothing equivalent for Mayorkas. There is no allegation that he committed any predicate act for human trafficking. Setting and administering federal policy on immigration is not human trafficking - even if a consequence of a policy is that human trafficking is more likely. That's why state legislators can't be prosecuted for aiding and abetting federal drug trafficking if they vote to decriminalize marijuana in their state. Acting in a legislative or regulatory capacity overseeing areas of law enforcement is not the same as engaging in the underlying criminal act, even if you make choices that will end up with more of the underlying criminal act being committed.

To get an indictment, you have to be able to provide some evidence to the grand jury that the accused committed the crime. Not a lot, but some. What's the evidence?