.22 WMR is a low demand cartridge. With all the demand for more popular cartridges, right now, it just isn't a good financial move to stop production, to gear up for .22 WMR.
It's roughly equivalent to Ford stopping production of their best-selling vehicle, the F-150, in order to run a couple thousand C-Max hybrids for the 7 people in the country that want one.
When the ammo companies get a chance, they'll run all of the .22 WMR they have orders for (and maybe a little more). But, there are plenty of other cartridges they feel they need to catch up on, first.
In addition, .22 WMR primarily uses jacketed bullets that are made on the same equipment used to produce bullets that are needed for centerfire ammunition production, as well as bullets that are in high demand for reloaders. Again, it doesn't make sense to stop producing a high demand bullet for ammunition production (and the component market), in order to satisfy a tiny fraction of the market with .22 WMR. ...especially when you have to shut down one of your ammunition lines, to produce the .22 WMR ammunition, once you do have the bullets.
One of my favorite rifles is a .22 WMR. But, I also know it's not a very popular cartridge. I buy large quantities when the price is right.
There's still some out there, and there will be more. But, getting your hands on some may require a little patience.