OK, so you know they aren't allowed. Here's why. An abstract method basically says "a subclass will provide the implementation". In effect the subclass is overriding the method, but what it's overriding is the lack of implementation.
Private methods cannot be overridden, because they are not visible to the subclass. So you can't have an abstract private method - it's impossible for a subclass to provide an implementation for it. Similarly, static methods cannot be overridden, and so it doesn't make sense for them to be abstract.
The bottom line is, if a method can't be overridden by a subclass, it can't be made abstract.