1. The Mueller team had all kinds of evidence against him that had been gathered through the FISA surveillance last year.
2. The FISA warrant is for intelligence gathering and probably cannot be used for criminal prosecution because of the shadowy nature of the process to secure the warrant for surveillance. Any evidence gathered this way would likely be considered inadmissible in court.
3. Therefore, the Mueller team needed to find other evidence through "legitimate" means that reinforced what was obtained through the FISA surveillance but would actually be admissible in court.
Renato mariotti has pointed out in the past this exact scenario...he is a former prosecutor..and now running for office
He also said that winning doesnt always mean a guilty verdict on Fisa cases