I was assaulted on a public side walk by hired security in Chicago

Location: Chicago, Illinois
I’m trying to understand why a detective would conclude there isn’t enough evidence to pursue criminal charges in an alleged strangulation case.

The evidence I have is:
My testimony.
An eyewitness who identifies the alleged assailant.
Photos of marks on my neck taken after the incident.
Video showing the alleged assailant at the scene, but not the alleged strangulation itself.
Video of the aftermath, including a bystander making comments suggesting the situation appeared life-threatening.
The alleged assailant admits he was present and involved in an altercation that day but denies strangling me.

The detective told me they can’t link the alleged strangulation to that specific person because the video doesn’t show it happening.

My question is: assuming these are the facts, what additional evidence would prosecutors typically look for to prove the case beyond a reasonable doubt? Is the absence of video showing the assault usually the deciding factor, or are there other types of evidence that commonly make the difference?

My concern would be to show up but then have this person smirking like he got away with it. I feel eyewitness testimony, my marks on my neck, and this person admitting he was there should be enough.

Bonus:The investigator stated I did not alert the police on site although I called 911 once I got home safely. And my story never changed. Maybe 1 hour after said incident.

Investigator also stated this will be looked down on by the prosecution because of that. He also stated although I was injured I didn’t visit the hospital. He said that significantly hurts my case. I don’t personally see any issues with it.

But I am feeling like I need more of a smoking gun. But maybe I have enough. I have my eyewitness..