I'm curious about the two attacks too. If the women can ID him as their attacker why wouldn't additional charges be filed? Or if his cellphone shows he was in the area. So many questions yet to be answered.
He also has a very distinctive beard that he's kept even while in jail that would be easy to identify if he had that during the previous attacks. Assuming he didn't try to change his appearance. Doubtful since he was dumb enough to carry a very traceable phone with him while committing murder.
Correct me if I am wrong, but, if I remember right from reading about the two other assaults, one woman was assaulted when the suspect grabbed her.
Not to downplay the crime, but, in the scale of things, it's not a very serious charge.
If he grabbed her and then dragged her into the woods, or threw her in his car for example, the charge would now be Aggravated Assault, which is a more serious charge.
If there was probable cause to connect the two crimes together, or the offenses were part of a scheme, then under Rule 9, a Judge could order the charges to be combined (Joinder Of Offenses)
I am not sure why we haven't heard anything about the other two attacks, but, it may be nothing more than if he was involved, and not having a record is true, to tie up a lot of time preparing a Prosecution/Defense for a "Simple Assault" charge that is a misdemeanor, and may add a fine of $1000 max, may not be worth it.
Bear in mind they also have 3 years from the date of the attack to bring separate charges, if they so decide to do so.
http://www.mass.gov/courts/case-legal-res/rules-of-court/criminal-procedure/crim9.html