They got him on wire fraud. Everytime a case has a shaky legal basis, they go for wire fraud. The legalese is stupidly broad, encompassing, and comes with huge penalties. [1] Up to 20 years in prison for any scheme to defraud that uses practically any digital tech at any point. It probably made sense in the 50s when wire fraud would have been quite a specific sort of crime, but in the era of the internet - now basically anything and everything can be construed to be wire fraud.
In Kelly vs US [1] a couple of politicians were convicted of wire fraud for closing certain lanes to create a traffic jam for political benefit. It was overturned by the Supreme Court but that gets into interpretive arguments of what the statute means when it says "OR". The law isn't like code. Anyhow, yeah the wire fraud statute really needs to be rewritten or broken down into more appropriate and targeted statutes. But that will never happen because it's a superweapon in its current state.