This is due to a myriad of reasons, but mostly due to rise of crypto currencies and mining. While not bad in and of themselves, crypto currencies require a large amount of processing power to be mined, which higher end graphics cards are excellent at doing.
Finding a graphics card on sale cheap, let alone finding a newer GPU at all, is going to be very tough for some time. As with the previous question of if pre-built gaming PCs are worth it, there are ways to circumvent the issue but they require a bit of patience and luck to achieve.
Stock alerts from your favorite retailers will help keep you in the loop as to when new stock arrives, but there is still the issue of dealing with auto-buyers and bots. So even if the GPU comes back in stock, there is still no guarantee there will be any available for purchase.
But stay diligent and check regularly, there is always hope!