These types of investigations move much slower than the 24 hour news cycle or social media. If the police haven't interviewed him yet it's because he is the prime suspect in a possible crime. Interviewing him would tip their hand.
You've been watching too much Criminal Minds and Law & Order. Tip their hands to what? There's not an "unsub" committing murders that they have to find from a collection of suspects. There's no mystery of the parties involved here.
An incident report from ALL the officers involved in the situation should have been the first step taken, but since that doesn't give everyone time to get their ducks in a row and formulate a cohesive story to cover their asses and throw out negative stories about the victim in the press, we're now almost three weeks in without his statement being given.
I'm not aware of whether or not an incident report was filed. I assume there was but I don't know. In any case if they were going to get there ducks in a row before submitting that report. The investigating officers almost always save the prime suspect for last. They want as much ammunition as possible before the interview, assuming that the investigating officers aren't actively involved in a cover up, which I doubt because of the intense scrutiny the department is under. The driver also undoubtedly has a lawyer that may very well be advising him not to speak. If true, that in and of itself speaks volumes.
Rest assured that no matter what BPD comes ups with that Maryland State police and the Justice department will be conducting their own investigations. The truth WILL eventually come out. Hopefully before the entire city is burnt to the gruond.