Simply put, they're very popular.
DNA 30s especially, because of the added capability. 30watts is a lot more then 20, plus the ability to fire some subohm coils. And, the two very popular choices, the Hana Modz and the ZNA, just so happen to be EXTREMELY in demand.
If its one of those you want, wait a while. The buzz will normalize and they'll be readily available. When Zenesis2 launched, it was impossible to get anything. No carto hybrids, no hybrids other than 18350 size, forget about seperate pieces.. but now? Basically everything in the Z2 lineup is available all the time, barring a few exceptions. The ZNA will be like that soon enough, Zen is working hard to crank them out asap.
You can get a Protovapor XPV, the lead time on them isn't too bad, and they're well liked DNA20 devices.
Also, if its not neccesarily the DNA board you want, but the power capability of 20watts, there are a couple other options. There is a few 22w mods out now, I think Sigelei makes one, and there is the Lambo 6.0, which goes up to 20watts,
ElectronicStix.Com · Lambo 6.0 VV/VW Device.
Simply put, DNA devices are popular, so they're hard to find. But options ARE out there if you do some searching.
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Food for thought..
Any mechanical device with a rebuildable atomizer, can surpass the power output of a DNA30. You can wrap the coil(s) to whatever resistance you like, and push more wattage than any regulated device can. As long as you use safe batteries and watch some tutorials on safe coil building, you'll be in good shape.