Legal Question in Criminal Law in California

Not necessarily, but sometimes.

Plea bargains are supposed to be an agreement where both sides get something in the deal. The DA gets a guaranteed conviction and the defendant gets a lower charge or lower sentence than if they went to trial and lost.

Or, it could mean that the DA is worried about being able to convict on the charges they've filed, so rather than risk losing completely, they're willing to offer a lower sentence or charge in exchange for the deal.

Sometimes. There are lots of reasons why D.A.'s make such bargains. Usually it's because they are just too busy to take all of their cases to trial. Other times it's because a trial would be traumatic for the victim or for some other witness. And yes, sometimes it's because there are problems with the evidence.

When you use the term "enough evidence" you may mean enough to convict. Usually, though, the question is not whether the D.A. can win but how likely a victory appears. If the D.A. thinks there is a good chance that the jury will reject some or all of the charges, she will have a stronger incentive to make a deal than she would if victory appeared likely. When the evidence is so weak that a victory seems impossible, the D.A. will often just dismiss the charges outright.

You should not infer that the D.A.'s case is unwinnable just because she offered a plea bargain. That is almost certainly not the case. Besides, if D.A.s only offered deals in unwinnable cases, defendants could always reject those deals and be confident of acquittal. That alone would be reason enough for the D.A. to make offers in plenty of winnable cases.