It is interesting what info is released, and what is not...
What are the reasons a person is killed like this...
Often, - woman moves into a new location, local takes notice kind of thing...
... there was a case (can't remember her name), woman worked as secretary in local FBI office, she just moved into a new neighborhood and was attacked and killed similar to Allison.. her's was a sexual attack. Took a long time to solve as while they had DNA, the couldn't match it... Turned out it was a guy down the block from her who did it. It was a one-time attack, so he stayed off the radar... Now, since Allison was not new to this neighbourhood, it might be that the is new. He might even be simply staying with someone, and then left.
Other reasons... what if she was PG, and there was fight over that?
In cases where new BF enters the picture, it tends to be the old BF who can't let go, rather than new BF - new BF tends to thinks relationship has a future.... but in this case, since LE has DNA, seriously can't see it being anyone LE is already aware of...