In fashion and footwear, stockouts are constant across the long tail of sizes and variants, because no shop can carry every option. Many customers who hit one simply buy elsewhere, often from a larger competitor.
The key insight is that out-of-stock is usually a coordination problem, not a supply problem: the product almost always exists somewhere — just not on that shop's shelf. Recovering the sale in real time, rather than restocking later, captures far more of the lost demand.