Let's look at the second "reason" first (that he lost more than he won). If prosecutors make good charging decisions, they are prosecuting people who are guilty. If the prosecutors do a good job of presenting the evidence against a guilty defendant, the defendant's going to get convicted. The fact that Lincoln lost more murder cases than he won merely shows that antebellum prosecutors in Illinois were making good charging decisions and presenting good cases.
Now let's look at the first "reason" (that Lincoln didn't try many murder cases). Unless you are a homicide specialist in a large jurisdiction like Miami or New York, you are not going to try many murder cases. Murders just don't happen as often as auto thefts. Having tried a few murder cases myself, I can say that the mechanics of trying a murder case aren't that much different than the mechanics of trying any crime of violence. There's a lot more riding on a murder case than on most cases, but that has nothing to do with the mechanics of putting on evidence. Lincoln was an excellent trial lawyer who could do an excellent job of defending a murder case.
As a matter of fact, Lincoln once won a murder case in the most spectacular way imaginable. Two brothers named Trailor had been charged with the murder of a man who had disappeared, and they hired Abraham Lincoln to defend them. The case looked grim. Despite the fact that the victim's body had not been found, the state had an eyewitness to the killing. Lincoln won the case by using the testimony of a medical doctor to prove that the victim was still alive. The victim surfaced shortly after the acquittal, and he never gave a satisfactory explanation of what had happened to him. The case was so remarkable that Lincoln actually wrote an anonymous newspaper article about it, and the article can be read in The Collected Works of Abraham Lincoln, Volume 1, pp. 371-376.
You don't win a murder case like this every day. The only other time that I know of where this happened was back in the reign of the Roman Emperor Constantine the Great. Not long after Constantine made Christianity legal in the Roman Empire, a doctrinal controversy broke out among the Christians of the Empire. The dispute took its name from the champion of the side which lost—a bishop named Arius. The view which ultimately prevailed was championed by Athanasius, the Bishop of Alexandria, who is sometimes called the Father of Orthodoxy. They played rough in those days, and Athanasius’s enemies trumped up a murder charge against him. An Arian bishop named Arsinius had disappeared, and the Arians were displaying a severed hand which they claimed Athanasius had cut from the body of his victim for use in making magic.
Eventually they complained to the Emperor, and he sent his half-brother Dalmatius to Antioch to preside over the murder trial of Athanasius. The legend has it that when the Arians displayed the severed hand at the trial, it angered the the spectators in the courtroom so badly that they almost lynched Athanasius on the spot. When it came time for Athanasius to present his case, two of his supporters brought a hooded monk out of the audience and presented him before the court. They took off his hood—and it was Arsinius. Athanasius had them display Arsinius’s hands, both of which were still attached, and then he asked the Arians whether Arsinius might have had a third hand. Dalmatius dismissed the charge in disgust. At least that's what the legend says.
What actually happened, as described by Athanasius himself, was that Athanasius’s supporters discovered that Arsinius was hiding in Tyre. They commandeered the man and brought him before Paul, the Bishop of Tyre, who wrote a letter to Dalmatius attesting that Arsinius was alive. The reality wasn’t quite as dramatic as the legend, but the letter of Paul was sufficient to get the charges dismissed against Athanasius. The full story can be read in Robert Bush Wheeler's biography, St. Athanasius: His Life and Times, pp. 98-100, and also in Historical Tracts of St. Athanasius, pp. 94-96.