Actually, we've got a pretty good idea why Oswald did it. The fact is, Oswald spent much of his life like a conspiracist. He wanted attention, and no one ever gave him as much as he thought he should get. He decided that being a Communist would get him attention. He defected to the Soviet Union to get attention. And when they didn't give him enough, he came back to the US. Marina didn't give him enough attention. He, not unlike Czolgosz and Guiteau before him, also wasn't exactly Mr. Mental Health 1963.
The fact is, only one Presidential assassination out of the four successful ones was shown factually to have been the product of a conspiracy. I've just finished reading a book about James A. Garfield, Charles Guiteau, and Alexander Graham Bell. And people think
Oswald was nuts--Guiteau had him beat solid. Guiteau was told by God to kill Garfield for the benefit of the Republican Party. (Proving that loony Christian Republicans aren't a new phenomenon, I suppose.) And there's absolutely no doubt that Guiteau did it, either, unless you accept his defense that the doctors actually did it. Which wasn't a bad one, given that Garfield probably would have lived if he hadn't been treated.
But you know what?
All forensic doctors who really know what they're talking about agree that heads jerk backwards under the circumstances in which Kennedy was shot, and it only takes a tiny, tiny amount of knowledge of physics and anatomy to understand why. You can learn about it in five minutes, but conspiracists never bother. The "magic bullet" shot has been replicated. You can watch it done, but conspiracists never bother. The Mannlicher-Carcano rifle was so successful that it was produced and used for
over fifty years, but conspiracists don't even bother to take the two seconds to look that up on Wikipedia. They just parrot people who didn't take the time to actually talk to people who know anything about firearms.
The fact is, you're asking the wrong questions. While an investigation may start with motive, only an idiot ends it there, because motive by itself is meaningless. As I've said many times, I have a theoretical motive to kill my mother. If she dies, I'll inherit a third of her estate. (Probably; I don't know about her current will.) However, I would never be put on trial for it, because there's no case against me. For starters, she isn't dead. But even if she were, you would have to do a lot more than just show I had a motive. You'd have to show that I was in California at the time. You'd have to show that I was capable of inflicting the lethal wound, whatever that was. You'd have to examine all the physical evidence, and my motive simply wouldn't be enough.
In order to pretend that Oswald's successful assassination of Kennedy is suspicious, you would have to ignore the fact that he had already tried to kill a convenient target. Which he had. Yes, Kennedy's death relies on the coincidence that Oswald worked somewhere on the motorcade route. But he had the job before the Dallas trip was planned, much less the motorcade route set. So why couldn't they have picked somewhere else, someone else, some other trip? Why pick Oswald out of all the world as a patsy?
Oh, and Abraham Lincoln's death? The result of a conspiracy.