They throw their stuff 10% off frequently, but why are the new wiz bang cameras cheaper than the old DSLR camera bodies?

Supply & demand. The old bodies are compatible with all the old lenses. The new mirrorless requires new lenses or a clunky adapter.

Because Nikon's mirrorless cameras suck.

Nikon is in a distant 4th place in mirrorless camera design and capabilities after Canon, Sony, and Panasonic. Their autofocus design has been sub par for years, their color science isn't as good, and their video capabilities are trash.

No one is buying their mirrorless cameras and those that do obviously aren't happy with them.

I’ll add DSLR tech is an interesting combination of mechanical and electronic engineering.

Mirrorless is pretty much 100% electronic in the body.

Circuits are cheaper than springs.

Nikon is way behind the curve on mirrorless, as they stake their "claim to fame" on DSLR. Witness Nikon(btw it is pronounced "knee-kon" in Japan) by keeping their legacy F-mount bayonet until recently, whereas Canon "bit the bullet" and discontinued their old SLR bayonet from the bygone film days. I was basically a Nikon DSLR man until a few years when Nikon discontinued their top of the line D500, I think that was the kiss of death for Nikon DSLRs. IMHO I think Nikon will stop making DSLRs after they sellout all of their remaining inventory of DSLRs.

Personally, I like my Nikon D7500 and D500. In low-light with flash, the ancient D7500 walks all over my Sony A6700 for photos, but then the A6600/A6700 are optimized for video and is aimed at vloggers.