Or there might have been more than enough evidence to convict him but the prosecutor offered him a deal with less time served in exchange for making the process easier on the courts.
You can fight a case through trial but if you’ve got bad facts you’ve wasted your time and money (unless you’ve got a PD) plus might just end up serving a greater sentence.
That’s not to say that having the capacity to afford serval attorneys on retainer isn’t beneficial, just that there is such a thing as a good plea deal when you’re stuck with bad facts.