Direct quote from the Judge's response:
"However, Smith will be permitted to testify generally as to the following: general background as to what the Federal Campaign Commission ("FEC") is, background as to who
makes up the FEC, what thc FEC's function is, what laws, if any, thc FEC is responsible for enforcing, and general definitions and terms thal relate directly to this case, such as for example "campaign
contribution""
He basically just wasn't allowed to give his personal opinion on whether or not Trump broke the law, which is all the defense wanted from him, so they didn't bother to call him.