I share your impulse to wonder why LE would hesitate to deploy this awesome tool. But I bet it will prove tricky to apply, or at least until LE and the legal system gets its arms around it. Remember the polygraph. In some ways, DNA face estimations would be less good than a witness sketch - DNA won't tell you how fat, old, hairy, tatooed, pierced or scared up a perp is - and we know that witness sketches are often of limited help, and sometimes mislead.
Yes, it could be a bit tricky with respect to certain traits, as they'd have to say that someone is "likely" this or like that. (For instance, someone with African ancestry is likely going to have dark skin color, whereas someone with Scandinavian ancestry is likely going to have quite pale skin color. But those things aren't set in stone.)
But there are some things that I'm pretty sure they can tell for certain with advanced DNA testing. Eye color, for instance. Simply being able to rule in (or even out) those with certain eye color would be very helpful. And eye color isn't something people can change unless they use tinted contact lens.
So, *if* the information in that article is accurate and if LE believes that the killer is still a threat (not deceased or in jail), I'm dumbfounded as to why they haven't done this testing sooner. Someone who kills so viciously in broad daylight is a major public safety threat, even if he targeted Vanessa.