You have no idea what evidence and or witnesses they have. You only know what was reported which could be true but it could also not be true. Sounds like they had enough.
I am not saying they don't have evidence. But prosecution didn't give him anything. There was no plea deal, just a plea. If it came with a shorter sentence or the most serious charge being dropped then it would make sense. The case was far from a slam dunk....there are plenty of people who would vote to acquit regardless of the evidence and he only needed one of them for a hung jury.