One of (at least) three reasons:
1. They want him gone.
2. They can’t prosecute Trump without throwing Joe under the bus.
3. They want to control the investigation and the narrative.
One of (at least) three reasons:
**********
4th......
They want to get VP Harris in as President. She then can
serve the remainder of this term if less than two years and
be re-elected to two four year terms. IMO that’s the plan.