Why buy a Pentax K1-II over Pentax K1?

This article explicitly discusses the differences between the 2. Whether or not those differences are worth any variation in price is entirely up to you.

https://www.pentaxforums.com/reviews/pentax-k-1-ii-review/introduction.html

Enter the K-1 Mark II, a new DSLR which is almost identical to its predecessor, but with the inclusion of several new elements:

• A new accelerator unit for improved noise handling at high ISO
• A Dynamic Pixel Shift mode allowing the capture of handheld pixel shift images
• Improved AF speed and tracking
• Increased ISO sensitivity (two stops added, up to ISO 819200)

The Pentax Forums site also offers a side-by-side comparison of features, and it really highlights that there are no differences other than what's listed above. https://www.pentaxforums.com/forums/pentax-cameras-compared/?c1=Pentax%20K-1%20II&c2=Pentax%20K-1

The Mark II was released 2 years later and wasn't intended to be a huge step up. It was a refresh and this actually resulted in being able to send in your K-1 and have it upgraded to the Mark II with some internal component changes. If you didn't own a K-1 and were shopping, it might make more sense to buy a more newly-manufactured camera. If shopping the used market, then I'd say it comes down to budgeting and what condition you can find the used ones in.