The simple answer is that a JD is a real doctorate, and holders of the degree can use the title. It is subject to the same state law guidelines as any academic doctorate.
Why it isn't done is not as simple. It isn't very common because it's only been about 30-odd years since the degree changed from an LLB (second bachelors) to a JD. Things moving slowly as they do, it just hasn't really caught on in common usage, guess most lawyers feel esquire is enough.
The ABA has released a statement (it's somewhere on their website) that the JD should be considered equivalent to the PhD for academic teaching purposes on the basis that it generally takes as many or more post bachelor's hours to complete.
I have seen the use of Dr. So-and-so, JD but that is generally in the academic arena, or sometimes in publishing. My best guess is that it is socially acceptable in the academic arena, not so much in day to day usage.
As to pretentious, many people think that someone with a PhD that calls themselves 'Dr.' outside of academia is somewhat pretentious (or so I've heard)