Good question. My reading of his work - including the Silmarillion - is that he tried as much as possible to write actual histories that could not be easily simplified into allegories. Readers naturally referred to his works as allegories, which he considered annoying because they were histories to him, grounded in actual history. Though they were also allegorical, they could not be simplified as to the specific allegorical messages they may have been, as his histories are built up from a past that extends hundreds if not thousands of years, in actual history. Middle Earth was thus - for example - it's own historical world that could not compare 1:1 with another history, but rather tha many other histories.