My guess is that this is due to the spike in traffic when the sale starts and they don't have the infrastructure scaled to take the extra load. As Knufen said,

My guess is that this is due to the spike in traffic when the sale starts and they don't have the infrastructure scaled to take the extra load. As Knufen said, this is not an insolvable problem, it just requires Steam to invest in infrastructure. In one of my previous companies, we had a standing mandate to be able to support traffic 2.5 times of the max peak traffic we had ever seen. It ensured that our systems didn't go down even under extreme stress but it cost a lot (and believe me we had a lot of folks complaining about it). So it is quite feasible that Steam simply decided that the cost of the additional resources is more than what they loose if the systems go down due to the load.