It's a stateless abstract class (i.e. it has no instance fields), it doesn't need a constructor because there is nothing to initialize.
It will, of course, have a default constructor which is compiler-generated, which any subclass constructor will call, but it will evaluate to call just the Object
constructor (which in-turn doesn't really do much: http://referencesource.microsoft.com/#mscorlib/system/object.cs )