Q:

Why can't a destructor be marked constexpr? 

In C++, you can declare many things as constexpr: variables, functions (including member functions and operators), constructors, and since C++1z, also if statements and lambda expressions.  However, declaring a destructor constexpr results in an error:
struct X {
    constexpr ~X() = default; // error: a destructor cannot be 'constexpr'
};

My questions:

Why can't a destructor be marked constexpr? 
If I do not provide a destructor, is the implicitly generated destructor constexpr? 
If I declare a defaulted destructor (~X() = default;), is it automatically constexpr? 

A:

As per the draft basic.types#10 possibly cv-qualified class type that has all of the following properties:

A possibly cv-qualified class type that has all of the following properties:
(10.5.1) - it has a trivial destructor,
(10.5.2) - it is either a closure type, an aggregate type, or has at
  least one constexpr constructor or constructor template (possibly
  inherited from a base class) that is not a copy or move constructor,
(10.5.3) - if it is a union, at least one of its non-static data
  members is of non-volatile literal type
(10.5.4) - if it is not
  a union, all of its non-static data members and base classes are of
  non-volatile literal types. 

Ques 1: Why a destructor cannot be marked as constexpr? 
Because only trivial destructors are qualified for constexpr
Following is the relevant section of the draft

A destructor is trivial if it is not user-provided and if:
(5.4) — the destructor is not virtual,
(5.5) — all of the direct base classes of its class have trivial
  destructors, and
(5.6) — for all of the non-static data members of its class that are
  of class type (or array thereof), each such class has a trivial
  destructor. 
Otherwise, the destructor is non-trivial. 

Ques 2: If I do not provide a destructor, is the implicitly generated destructor constexpr? 
Yes, because implicitly generated destructor is trivial type, so it is qualified for constexpr
Ques 3: If I declare a defaulted destructor (~X() = default;), is it automatically constexpr? 
Indeed, this destructor is user-declared and implicitly-generated and thus it is qualified for constexpr. 

I'm not able to find any direct reference that only trivial destructors are qualified for constexpr but if the destructor is not trivial then it is for sure that class type is not cv-qualified.  So it kind of implicit as you can't define a destructor for cv-qualified class. 

C++20 Update
Since C++20, user defined destructors can also be constexpr under certain conditions.  
dcl.constexpr/3:

The definition of a constexpr function shall satisfy the following
  requirements: 

its return type (if any) shall be a literal type;
each of its parameter types shall be a literal type; 
it shall not be a coroutine ([dcl.fct.def.coroutine]); 
if the function is a constructor or destructor, its class shall not have any
  virtual base classes;
its function-body shall not enclose ([stmt.pre]) 
  
  
a goto statement, 
an identifier label ([stmt.label]), 
a definition of a variable of non-literal type or of static or thread
  storage duration. 

A:

If what you're looking for is reasoning behind the restriction, have a look at this paper which clearly states that the restriction is artificial - there is no intrinsic property of destructors that prevent them from working in constexpr contexts, and indeed compiler implementors agree that supporting them in constexpr contexts will be trivial to implement. 
I guess the C++ standards committee originally placed the restriction in C++11 because they didn't want to deal with destructors at that time and it was easier to just rule them out entirely. 

A:

A destructor can't be constexpr because constexpr functions can't have side effects and destructors by definition are only useful through side effects.  In short, it would be useless to have a destructor that is constexpr. 
A object cannot be constexpr if its destructor is non-trivial.  A defaulted one, if trivial, will be considered constexpr
Live
From [class.dtor]

Each decl-specifier of the decl-specifier-seq of a destructor declaration (if any) shall be friend, inline, or virtual. 

Missing from it, constexpr.  So you could just take it as: because the standard says soTM

