It's exactly economies of scale and support. Pi is still pretty tiny as a business and benefits from community goodwill and input. Only now are they achieving the millions of units shipped which are needed to achieve good economy of scale.
To be fair, the tinker board is only 50% more and does include additional features (some extra speed and GbE). The US release is happening right now, so that could affect availability worldwide as Asus tries to launch with sufficient quantities. I somehow was able to order one, sold by Amazon US no less, and get it last week; there's not even a listing anymore.
Not too bad for a product that was announced late January.
That doesn't explain the zero's price. They are making a loss on those, and a quite big one at that. It is not possible to make something like the Pi Zero for less than double it's retail price.
There is e.g. the Asus tinker board which is twice the price and out of stock. https://www.asus.com/uk/Single-board-Computer/TINKER-BOARD/