They had plenty of proof and eyewitness accounts to back it up as well.
The trial, and the jurors, said differently. It wasn't as cut and dry as you seem to think. There were no witnesses to the shooting, otherwise they would have taken the stand. The witnesses that did take the stand had conflicting accounts. The things the jury focused on was on the manslaughter instructions from the judge, because the prosecution hammered away at the fact that Zimmerman harbored hatred and ill will against Martin. But they weren't buying the notion that Zimmerman held hatred or ill will against Martin, so they dismissed the second degree murder charge as being applicable.
The jury then asked the judge for specific clarification on the manslaughter charge, though, which is a clear indication that that's what they were focusing on and were considering returning a guilty verdict for the lesser charge. If it was as clear cut as you think, after 8 hours of deliberating the jury would have simply returned a not guilty verdict, rather than ask the judge for manslaughter clarification, and then continue deliberating for hours. They were probably closer than you think to finding him guilty of manslaughter, but the prosecution's case was emotion-based, full of wishful thinking and the logical fallacies of asking questions to try and get the jury to assume a conclusion rather than present facts that are not subject to assumption.
The prosecution also constantly referred to Martin as a "child" rather than the young man he was, which is a blatant attempt to influence the 5 mothers on the jury, and the jury likely took offense to that. Instead of being presented with facts, the jury likely felt the prosecution was trying to play them.
All it takes for manslaughter is a preponderance of the evidence, rather than beyond a reasonable doubt. They had that (based on Zimmerman's own statements regarding the nearly 6-minute interaction between the two where Zimmerman could have avoided conflict and did not do so), but the prosecution muddied the waters so badly with their emotional "what ifs" and "how abouts" that by the end of the trial the jury (and everyone else) were less clear about what took place at the initial contact between Zimmerman and Martin than they were at the start.
There was no mountain of evidence that exonerated Zimmerman in any clear fashion at all (the jury's question on clarification makes that evident), nor was there enough evidence to convict him, which is why the case never should have seen the inside of a courtroom.