The reason they are autistic is that they see a million and one reasons not to be sentimental, not to "get personal". When they open themselves up to this possibility, they get exceptionally personal.
Think of it this way: suppose that you created a system of thought more complex and quite frankly amazing than anything else that had come before it. You were able to accomplish this by completely distancing yourself from your own fears and values, and so have created something that is alien to typical human concerns. Most people can only barely understand you, if at all. Now let's say that you all of a sudden choose to express an opinion which reflects how you really feel. There is a good chance that the people who barely understand your ideas will feel exceptionally threatened by your opinion, because how are they to know that it isn't an alien production of your (typically detached) perspective? To understand your perspective, must they first adopt your opinion?
In the 1933, Einstein tried to immigrate to the U.S. (to escape Nazi persecution). Unexpectedly standing in his way was a group of angry women's rights activists who were upset about comments he had made about women being "over fussy" and "in need of proper management" by their husbands. This was only the second time this had happened: in 1924 he had gotten in some serious hot water with Amercan editorialists by arguing that American men "did not properly discipline their women". It was a paradox of his genius that this man who could see deeper into the processes of nature than seemingly anyone else had, with respect to all matters of personal conduct, a decidedly chauvinistic attitude belonging to the 19th century, as opposed to the 20th. I think this peculiarity can be explained, though, in the fact that the shadow is always opposite ideologically to the ego: an ardently progressive reformer like himself would be expected by have a shadow that is static in every sense of the word. Einstein often described himself as the tool by which God investigated the question of whether or not he had a choice in creating the universe. His belief in a god (although not the Yahweh of his ethnic faith) eventually led him to insist on the incompleteness of quantum mechanics, which eventually led to his falling away from the mainstream in physics. Certainly it never availed Einstein to share his personal beliefs about anything, because his friends always had personal reasons of their own not to listen.
One person who did listen to Einstein was the very conservative and authoritarian Sigmund Freud, in whose work Einstein took a great deal of interest. Einstein's person was also "borrowed" on behalf of all sorts of causes: people frequently attempted to exploit his fame. That may be another reason for his reclusivelness, in that he may have feared people close to him were trying to use him.