They arrested them because their creation ruined their plans for mass surveillance, and the conviction was more of a revenge on the part of the government and the banks because now many people will want to use banks less, making it more difficult for them to collect more taxes.
On top of that, they prevent people from learning about Monero, because if more people start using it, they will have less control over it. Besides, Zcash surely has methods to be tracked, but they don't want to use them yet until everyone is already using it on a daily basis.