They're ramping up production. They hope to have 25 units available on launch day, and 1 available each consecutive day until everybody that wants one either has one or has given up any hope of ever finding one (99.999% of potential customers will end up in the latter category).
I was in full "shut up and take my money" mode when they announced the NES Classic. I do not own one because at first I couldn't find one, and now I'm so pissed at their continued "desirability through rarity" marketing that I may never buy another Nintendo console. (For reference, I own every stand-alone Nintendo console from the NES to the WiiU...have not bought a Switch).