There were several reasons why Saki chose to have Framton Nuttel suffering from a nervous condition. In the first place, it helps to explain why this city dweller is spending time in the country. He needs to relax in a quiet, peaceful setting. This, of course, is ironic because he is going to be scared out of his wits. Secondly, the fact that Framton is suffering from a nervous condition makes him particularly susceptible to the practical joke that Vera plays on him. He is already jumpy, and it wouldn't take much to set him off. Thirdly, his nervous condition is a felicitous way for the author to characterize him; it makes it easy for the reader to visualize the unhappy stranger sitting on the edge of his chair, tugging at his collar, trying to pay attention to his young hostess, and so on. The main point of the story is not just that a man is frightened by what he thinks are approaching ghosts, but that a man who is seeking a rest cure for bad nerves runs into such a shocking experience in what should be, and actually is, a peaceful English country home where tea time is one of the most exciting events of the day. A fourth reason for Saki's making his main character a nervous hypochondriac was that it makes the story amusing rather than frightening. This is not a horror story but a humor story spiced with a bit of the uncanny, and as such is characteristic of much of Saki's short fiction.

In "The Open Window," why does Saki give his main character a nervous condition?

I have to admit I have one big issue with your question, and that is that I do not think that Framton Nuttel is actually the main character. Although we certainly start with seeing his perspective in the story, it very quickly becomes clear that the main character is the storyteller par excellence, Vera, who is able to deceive and trick Mr. Nuttel with consummate ease and then does the same to her family.

And this really is the reason why Saki chooses to bestow upon Framton Nuttel this "nervous condition." This explains his susceptibility to the story that Vera weaves and explains his quite sudden and shocking reaction when he sees the "ghosts" return from their hunt. As one of the returning hunters comments, when he saw them, Mr. Nuttel "bolted" out of the house straight away, allowing Mrs. Sappleton the opportunity to call him an "extraordinary man."

Thus Saki gives Mr. Nuttel his "nervous condition" to show how open he is to being deceived and also perhaps to question Vera's intentions. On the one hand this is an incredibly amusing story, yet seen from a different light, it is an exercise in cruelty.