I started to write this out but realized a more polished explanation might be more useful:

"Snorkels constitute respiratory dead space. When the user takes in a fresh breath, some of the previously exhaled air which remains in the snorkel is inhaled again, reducing the amount of fresh air in the inhaled volume, and increasing the risk of a buildup of carbon dioxide in the blood, which can result in hypercapnia. The greater the volume of the tube, and the smaller the tidal volume of breathing, the more this problem is exacerbated. Including the internal volume of the mask in the breathing circuit greatly expands the dead space."

https://en.wikipedia.org/wiki/Snorkel_(swimming)

(The face mask snorkels are relevant to the "greater the volume of the tube" part.)