A guided party that turns back a short distance from a summit is following a rule agreed before the climb began. The reasoning behind turnaround times is consistent across mountains.
The descent is the dangerous half
A substantial share of serious mountain accidents occur on the way down rather than on the way up, when climbers are tired and concentration has faded.
Fatigue accumulates through the ascent, so the party attempting the descent is measurably less capable than the party that set out that morning.
Planning must therefore reserve capacity for the return, and a summit reached with nothing left in reserve is a summit reached too late.
Turnaround times are fixed in advance
A turnaround time is set before departure, based on the route, the party's pace and the hours of daylight available.
Setting it in advance is the point, because the decision is made while everyone is rested rather than at altitude with the summit visible.
Judgement deteriorates with altitude, cold and exhaustion, which is precisely why the rule is designed not to require judgement when it is applied.
Weather windows close faster than they open
Mountain weather deteriorates rapidly, and afternoon convection, rising wind and cloud building on a ridge are all common patterns rather than surprises.
A route that was straightforward in the morning can become serious within an hour once visibility drops and fresh snow covers the tracks.
Guides read those signals continuously, and a decision made on developing conditions will look premature to clients who cannot yet see what changed.
Objective hazards are time dependent
Snow bridges over crevasses, ice-covered slopes and loose rock faces are all more stable in the cold hours before the sun reaches them.
This is why alpine starts happen in darkness, and why crossing certain sections after a set hour is treated as unacceptable regardless of progress.
Rockfall and avalanche risk follow the same daily cycle, so a late party faces a mountain in a different and more hazardous state than an early one.
The commercial pressure runs the other way
Clients have paid a large sum, travelled far and may have one attempt available, which creates real pressure toward continuing.
Reputable operators handle this by making the turnaround rule explicit at booking, so it is understood as a condition of the trip rather than a failure of it.
An operator willing to negotiate that rule on the mountain is offering a materially more dangerous product, and it is the single most useful thing to ask about before booking.