Why did Mike use a bank to pay his people in Better Call Saul? Was not this his undoing, using a lawyer using a legit institution? Why not have someone else use multiple dead drops which was the normal MO for the drug trade anyway, something that worked and he was familiar with?
Dead drops only work when the people you are hiding them from don't know who is picking up from them. Mike knows this. Mike knows how the police work, how investigations work, and particularly what raises police suspicion.
All of the families of "Mike's guys" were known to the police. With an ongoing lack of cooperation from the incarcerated men, some entirely legitimate and even expected surveillance on the families would quickly have resulted in the dead drops being discovered. Then it would only be a question of the police watching the dead drop to see who was dropping the cash off.
A further complication is that cash changing hands to the families of people awaiting trial for being involved in a massive drug empire through dead drops is going to be a massive red flag. The police are going to assume (quite rightly in this case) that it's hush money of illegal origin, due both to the identity of the recipient and the manner in which the money is transferred.
Whereas, there is nothing overly suspicious about a lawyer visiting a bank and accessing some safe deposit boxes. Nor is there anything obviously untoward about the families visiting a bank. Even if it was all of them visiting the same bank; they were all connected to Los Pollos Hermanos through legitimate employment (even if there was illegal activity on top), and there may have been reasons that they all banked with the same company.
If it weren't for Hank choosing to do the legally dubious thing of putting a tail on the lawyer for the defendants, law enforcement would have known nothing.