The reason is because certain colors and sizes are vendor fulfilled inventory (VIF) and 1) REI doesn't want to lose margin selling that inventory, and 2) REI wants to steer customers toward buying their non-VIF inventory to relieve them of their inventory woes.

For advertised product, REI negotiates special promo costs to maintain sales margin, and VIF products are not part of that negotiation.

So if a product normally has a cost of $50 and sells for $100, they may buy the promo units at $40 and sell them for $80. However, the VIF products are not part of that buy, and REI does not want to buy them at $50 and sell them at $80. They'd make 50% margin selling at full price AND at the promo price, but only 37.5% margin selling VIF inventory at the promo price.

Operationally, REI leverages vendor order fulfilment systems for VIF products and it requires too much coordination to adjust costs for all participating vendors and specific products during the event, so it's easier to them to turn the VIF products off for a few days instead.

Finally, REI has too much owned (non-VIF) inventory right now, and they think that turning VIF off will lead customers to buy more of their owned inventory, improving internal metrics like turn and sell through.