That’s apparently because the 5800X3D was discontinued last month. The 5700X3D is newer even though it’s slightly slower, so it hasn’t been discontinued yet.

(Incidentally, I found this all out a few days ago and decided I’d better finally upgrade my 1700X while I had the chance, so I’ve got a 5700X3D sitting on my desk waiting to be installed as I type this. Microcenter has them on sale for $180 right now, BTW.)