you have to understand the difference between wholesale and retail. when you buy a tv, about half the price covers the wholesale cost of the tv to the retailer, and the other half covers the retailer's overhead (e.g. employee wages, building lease, utilities, debt service, shareholder profit).
the law of supply and demand applies to both sides of that equation, but you seem to only look at the wholesale demand. yes, on black friday, customers demand a lot of products, but they demand very little retail service. they tolerate crowded stores, long lines, distant travel and terrible hours (one day a year!). you can see the same principle in retail where larger packages are cheaper per unit (e.g. the 2-pack is almost half the price of the 10-pack). from a wholesale perspective, the more you buy the higher demand. but from a retail perspective, the more you buy per package or per visit the lower your demand for retail service.
wholesale demand will drive up prices only slowly. you can't build, tool and staff a factory for one day and then tear it down the next (yet....). so, higher demand only drives up wholesale prices if it lasts for months or years. so, on black friday, wholesale prices are generally stable, but the retail margins drop precipitously. if a retailer were to maintain normal margins on black friday, they would be lucky to get normal volume and just barely break even. however, if a retailer cuts their margins in half on black friday, they are likely to more than double their normal volume and more than make up for the margin in volume.
it is worth noting that high-volume/low-price retailers have a hard time staying open all year. if a retailer were black-friday-busy on some random tuesday in february, many customers would simply find somewhere else to shop. for example, many consumers avoid walmart, despite the low prices, because of the long lines. that said, other people do frequent walmart. there are in fact retailers like "weekends only" that claim to offer lower prices specifically afforded by their reduced retail service (not open during the middle of the week).