In spite of what you may have heard on Fox news, it isn't a simple matter of forgetting to file a few forms (yes, I heard some ditz say that):
https://www.nytimes.com/interactive/2017/10/31/us/politics/manafort-mueller-money-laundering-fraud.html
The DOJ didn't charge him with income tax fraud, because they've got him without that (and with income tax fraud, they would have to prove intent - even though we all know there was intent.)