short answer: plane was going too fast to stop.

long answer: we won't know why until the National Transportation Safety Board (NTSB) does its investigation.

but from what information is known is that the pilots (the plane was being operated by 21 air, amazon does not have pilots) were coming in too fast and unstable, and ended up touching down halfway down the runway. it appeared that the left main gear was still not on the ground at at least ⅔ down the runway, which might have not allowed the reverse thrusters out.

it could potentially be due to winds from an incoming thunderstorm, if you look at the footage there are some dark clouds in the background. it looks to me that crew tried to stick the landing from a fast and low approach, instead of powering up and going around. but that's just with the information i have and again, we won't know until the NTSB releases their preliminary report

What we know is that the plane didn't land and instead hovered. This could be due to ground effect. Ground effect is an effect where planes have significantly more lift when within a wing length of the ground. Usually Pilots can mitigate this but sometimes they make a mistake and float down the runway. It's even worse when they make the plane go up and down at that low altitude. That's called a pilot-induced oscillation.

There might have also been a microburst involved. That would make it seem that there's headwind up until a certain point and then there is a tailwind. That means that at the airport, The wind speed would look fine up until a certain point where the direction reversed making it even harder to land. From that point, the pilots should execute what's called a go-around and basically take off again but there might not have been enough Runway to do so.

This is speculation on my part. There may have been other things involved but this is my best guess. I think the microburst is the most likely and they may have been coming in a little too fast regardless.

There can be a number of human factors that create errors on landing. Some people just want to get out of the plane as soon as possible and so they are more willing to take risks. Sometimes they have complacency because they've landed at that same airport hundreds of times. Maybe they were fixated on some other problem in the cockpit while landing. All of that should be uncovered in the investigation.