Speaking from a Law Enforcement perspective, and this isn't meant to be an excuse, but sometimes, we don't have all the info people assume we have.
Sometimes, departments are reluctant to share info with other departments, , I have seen it first hand, that coupled with botched investigatory practices is sometimes, why investigations stall.
The media can also be a HUGE pain in the



as well, there are times, when things are put out in the media that aren't true or aren't completely true about the case, which makes it appear that since the public knows it we obviously have more info on the case, which is not only untrue, but adds undue pressure .
Then there's also always the issue of media getting ahold of and leaking critical info that law enforcement doesn't want released, which has lead to criminals actually being able to avoid apprehension .
Now within large metropolitan areas.. Philadelphia pa for example, there can be so many homicides at one time that a department can get overwhelmed trying to solve them all, which means some will go unsolved, it may be for long periods of time, sometimes, the don't get solved.
Now as for the victims,
Its certainly more than coincidence, like I said with the info given Id be willing to take a guess.. without seeing ANYTHING to generate any linkage findings, you are looking at 1 possibly 2 Serial offenders operating in the same area.
But without the required materials Id be hesitant to state whether or not I felt more were responsible, or that I thought the possible suspects were responsible for more than they may be.
You'd have to look at the minute details in each case, to determine whether or not this was the work of repetitive predator
Unless he was to be caught and admits to certain crimes, which we love.
Other factors also come into play in cases of multiple homicides as well, socioeconomic factors like the crime rate , the unemployment rate in the area, the drug culture, in the specific area etc..
It sounds as if it was so many so fast that Law enforcement got logged under, and couldn't link cases, before they went cold .
Which is sometimes a sad truth of the business