I'm going to hold off judgement until the coroner's report gets released. Could easily be this guy was high on crack or PCP which accelerated his heart rate, leading to him having a fatal heart attack while on the ground.
A big guy like that who is flying on some kind of upper can be difficult to restrain, even for four officers. There have been guys on PCP who kept on fighting even after having been shot several times. It's easy to say "Why didn't the cops do X or Y" when you've had no experience in law enforcement. Pinning the neck like that is pretty standard because it's usually pretty safe and it makes it more difficult for the perp to get leverage to either run or keep on fighting. A perp can still be a threat even with his hands cuffed behind his back. Officers have lost their lives to cuffed perps, which is why tactics like pinning the neck started being used in the first place. Pinning the back can actually be more dangerous than pinning the neck because the perp's lungs and diaphragm are being compressed, making it difficult to breathe. As others have said: if he can talk, he can breathe.
"Why didn't they use three cuffs and hogtie him?" Firstly, cuffs for the wrist are not designed to go around ankles. It's extremely painful and often the loops won't fit. That's why they make shackles. But hogtying is usually only used in limited circumstances because many people see it as dehumanizing. It can also lead to injury of the perp either from them fighting their restraints, or when lifting the perp to place them in a vehicle. Broken bones, dislocated joints, lacerated skin, potentially dropping the perp...then you've just given them ammunition to get out of their charges and sue you and your department.
