Exactly what he did? "sexual harassment" is an extremely broad category. Was it a few sexist remarks, or something much worse?
http://web.mit.edu/policies/9/9.5.html
Here's the overview of sexual harassment:
> Sexual harassment may take many forms. Sexual assault and requests for sexual favors that affect educational or employment decisions constitute sexual harassment. However, sexual harassment may also consist of unwanted physical contact, requests for sexual favors, visual displays of degrading sexual images, sexually suggestive conduct, or offensive remarks of a sexual nature.
As to why MIT doesn't get into more detail...that's kind of the modus operandi for these kinds of internal, non-criminal investigations. I don't know what you're reading into this, but perhaps you think the professor has been harshly punished for a few ambiguously sexist remarks, and you think seeing them would vindicate him? Perhaps. But it's also possible that the professor did something egregious, and it may not be helpful to air that out (especially if it means reproducing offensive photos that were allegedly sent).
So when is conduct benign enough that the University could just publish, and when is it not? That's a difficult decision. But for the most part, if the purpose of a press release is to explain why online materials are no longer online, it's not necessary to get into the nitty gritty of the findings of the investigation. If the professor wants to publicly fight the investigation, then it'd be more appropriate for the university to reveal the damning information it has.
The only issue here is lack of (public) evidence.
Extremely vague.