Why the HttpServlet class is an abstract?

Why would HttpServlet not be abstract? An instance of just HttpServlet would be useless - the whole point of a servlet is to be able to provide useful responses to requests, and HttpServlet can't do that. It's generally a good idea to prevent useless situations where possible.

As another example of something similar, consider MouseAdapter. That's abstract despite having no abstract methods, because it's useless until you override at least one method.