These are some of the reasons given in the press at various stages by trainers and jockeys why their horse did not win.

Not fit enough.
Not suited to the track.
Cannot handle the track conditions.
Is unwell (as opposed to race fitness).
Medication may have an adverse effect.
Hit with a clod during running.
Bad ride by jockey.
Bad barrier.
Not suited by the distance.
Pace of the race did not suit.
Checked during running.
Could not get clear in running.
Injured during running.
Could not carry the wieght.
Did not respond to the style of riding.
Days temperature was too hot.
Windy day irritated the respiratory system.
Saddle slipped during running.
Gear failure (tongue tie).

Can you add some more please?

The reason I've added this post is to show how the very best of selection methods can have a run of outs without actually being flawed.