Cultural conditions for quality
The consumer wants as little waste as possible. If they see stems and know that they will not be tender and succulent like the blade, they will understand that the stems represent product of poor quality. Of course, the grower's packers can trim them off, but then it is the grower who suffers from the poor quality. The lower photo also shows long stems, while the upper photo shows virtually no stems. Stem length is not a matter of timing, but a matter of cultural conditions.
Two factors control the stem length, light and ventilation. The natural habitat for oyster mushrooms is on recently dead trees or branches in the forest. Sometimes the branches, that the mushrooms grow on, fall to the ground and are covered by leaves or other debris. If mushrooms are to distribute their spores, they must grow above the debris. Both light and low carbon dioxide are signals that the mushroom is in the open where it can distribute its spores. Some of the debris will be rotting and releasing carbon dioxide, so carbon dioxide is usually found in high concentrations on the forest floor. Winds, natural ventilation, remove carbon dioxide, but can have little effect on that in the debris. In seeking the low carbon dioxide levels, the mushrooms escape the debris. In nature, winds serve a second purpose, they help distribute the spores; that is one purpose the grower wants to avoid. He must avoid it by timing the picking.