Why can't we autowire static fields in spring?

Because when the class loader loads the static values, the Spring context is not yet necessarily loaded. So the class loader won't properly inject the static fields in the bean and will fail.

Because using static fields encourages the usage of static methods. And static methods are evil. The main purpose of dependency injection is to let the container create objects for you and wire them. Also it makes testing easier.

Once you start to use static methods, you no longer need to create an instance of object and testing is much harder. Also you cannot create several instances of a given class, each with a different dependency being injected (because the field is implicitly shared and creates global state - also evil).

According to OOP concept, it will be bad design if static variables are autowired.

Static variable is not a property of Object, but it is a property of a Class. Spring auto wiring is done on objects, and that makes the design clean in my opinion. You can deploy the auto wired bean object as singleton, and achieve the same as defining it static.