Supply/Demand.
The 6700K is older, no longer being manufactured (AFAIK)... but it's still an extremely viable upgrade for someone with an i3 etc of the era.
It's been that way for years. Try finding a 4790K (new!) today! It's kind of a moot point, considering you could "upgrade" to the 7700K without replacing the board etc, but it's just the way the market for end of life CPUs go.... at least for a while.