Why is Jonas a male in The Giver and not a female?

The short answer is that we don't know. That would be a question that the author, Lois Lowry, would need to address herself and I do not recall her ever mentioning that she had considered making his character female instead. 

What might be interesting to consider is whether or not the gender of the new Receiver is important and whether the new Receiver being a female would have changed the storyline. In some books we can easily see that a female character would drastically change the storyline, but I am not sure the argument for that could be made regarding The Giver. 

Males and females in the book are treated, in some ways, very similarly. That is to say, each gender works, takes care of the family, dresses similarly, and because they are a sexless society, there are not the same traditional male and female tensions we have in our own society. 

Also, there was a previous Receiver of Memory who was a female, and she reacted in much the same was as Jonas did once she began to receive troubling memories. Although Jonas managed to receive more memories before he finally stopped, he stopped not because of the memories as she had, but because he learned what "release" truly was. In both cases, however, the role of Receiver caused them to learn things about humanity that caused them to want to stop being the Receiver. So, one could argue that male or female would have trouble with becoming the Receiver, and that gender might not play that significant of a role.