"Why - in particular - is transporting a minor across a state line for sexual purposes worse (or not as worse - "
This is not a moral issue, it's a direct result of the Federal structures at the at heart of US government. States have large latitude regarding what happens within their state.
Do something bad in your state, and you have to deal with your local police and courts. This matters. Trayvon Martin's killer would not have walked away a free man in most states, but he had the good fortune to be tried under Florida's "Stand your ground" laws.
However, crossing a state line transforms an act from a state issue to a federal issue, so different, larger, much better resourced organizations can get involved.
In particular, the FBI and US Attorneys ( an elite group of powerful prosectors) can get involved, and the case will be tried in front of a federal rather than local judge.
For the larger and more complex cases, often only "The Feds" have the resources and expertise to pursue the case, so local prosecutors will often look for some way to transform a difficult case into "a federal case".
This has a certain logic to it. How many resources can, say, the Bangor, Maine devote to child pornography relative to the FBI?