When the high command is about to shut down the Season 1 case on Avon Barksdale, McNulty comes up with taking the case to the FBI and he goes to them along with Daniels and Freamon to tell them about it. They end up discarding working with them because the FBI's only interest is pinning the corrupt politicians through the drug kingpins' cooperation, meaning reduced sentences. Daniels etc are not willing to see that, they want full weight on the Barksdale organization, so they walk.
My question is why not take the case to them? With the raids causing the wire to be shut down and the case ending from unwillingness of high command, the sentences are going to be very weak on Avon and most of the rest anyway. So if they are getting shorter sentences than they should in any case, according to Daniel's detail, why not go to the feds anyway? They are going to get the drugs plus the politicians.
Bonus question: earlier in the season, before Davis start pressuring Burrell, the case is constantly exhausted from high up the chain of command, they just don't want Daniels and McNulty to dig into it and they, Burrel, Rawls, etc, just want it to go away fast. But why? Why wouldn't they want to build such a big case to take down a criminal organization?
And last question, related to the previous one: what leverage do the corrupt politicians (Davis) to be able to call the shots on what the police investigates or stops investigating? What stops Burrell and others to just dismiss the politicians' demands and do things right? Are they getting bribed?