Stationery stores not stocking in demand items

Increasingly a few stationery stores in panaji are not stocking quick selling items like staples for staplers. Almost every person requires staple pins for staplers, yet a stationery store did not stock staple pins of the commonly used sizes, it only had staple pins of the large size.
Similarly u clips are widely used, and the store did not have the plastic or other clips in stock
Reasons why they are not stocking these items will be appreciated.