Pleading insanity is a real gamble for a defendant; it means he has completely given up on an alibi, on denying he did it, on suggesting that there was justification or mitigating circumstances, etc. He is openly admitting that he committed the murder and his one and only defense is that we should take pity on him because we are willing to believe he is sick.
This defense very seldom works in the real world. In addition, there are are least two conflicting standards - the old M"Naughton Rule that he was so crazy that he didn't even know what he was doing - and the newer Durham Rule that he knew but really really really couldn't stop himself. Either one is a real high hurdle.
Considering that the 'reward' for a successful insanity plea can be a lifetime locked in a psych ward - where he would have even fewer legal rights than if he were in prison - this defense is trotted out only for cases with a life or death sentence. Frankly I think a lot of jurors are saying to themselves, "I don't care that he hears voices or sees phantoms. I don't want this guy ever turned loose even twenty years from now. Since I don't trust bleeding heart shrinks, I'd rather send him where he'll be locked in a cage forever or put to death."