Trial By election

Similarly, the president cannot be prosecuted for firing the FBI director, no matter the cause. If Trump accepted a bribe to fire Comey, that would be illegal. But the firing itself would not be, because executive branch personnel decisions are the purview of the president.
Maybe you think this is a legal loophole. But that doesn’t matter. The justice system is based on rule of law as written, which means Mueller probably can’t indict Trump for obstruction.
Indictment v. Impeachment
The constitutional provision for removing a president is impeachment. Not charges filed in a court of law, which may not be possible. Nor the 25th amendment, which is designed for when the president becomes incapacitated by a heart attack or stroke.
Meuller’s team knows this. Trying to indict the president is risky. It could easily fail. And filing charges would trigger a constitutional crisis. They need a better strategy.