Why Couldn't They Kill Harambe With Non Lethal Force (BLM'ers As Well)

Because it's very difficult to nonlethally incapacitate someone with a gun without getting shot particularly if they are at range. Fear, the need to make split second decisions and the simple fact that guns despite what some cop procedurals would have you think are difficult to aim under stress and most cops aren't former snipers or Olympic sharpshooters. Shooting someone in the arm to disarm is also a laughable suggestion often brought up. It's a great way to miss and get yourself shot instead,

Even "nonlethal" options can kill. Brain damage or iternal bleeding from baton hit can kill or leave one brain dead. Tasers can cause heart failure or other complications, pepper spray can cause allergic reactions or trigger a severe asthma attack. Even a bean bag gun can kill in certain circumstances. Gases have limited places in which they will be effective and not a danger to the cops or nearby innocents as well. Similar problems apply to flashbangs. Also flashbangs can and have started fires and caused burns and even a few deaths. Nonlethal weapons are occasionally rendered less effective on people who are high on potent drugs or in a "adrenaline rush"

Guns are the best balance of cost, flexibility, stopping power and range for police to use though all other options are viable in certain situations sometimes in tandem with one or more of the others including guns. 

Believe me most cops would love a ranged nonlethal option as good as a gun at incapacitating. But it's not realistic at the moment.

This is by no means a complete or professional analysis but I believe it is a decently informed view.