Gun handling properties, and then fit and finish gentlemen!
We seem to forget that this was first and foremost to the shooters who wanted the best. Barrels struck well, ejectors nicely timed and true of balance. Second was the gun's over-all quality.
The great Westley Richards, made 9 out of every 10 for export, and for the rough. Their Sle gun was made by AA Brown!
When lesser quality guns are your bread and butter, how can they be counted on for your "Best gun."
Same with the mass-producer, W&C Scott - in today's world, they'd have moved to China, or Turkey - already.