i would imagine it's due to a combination of factors, such as the general rise of cost for just about anything these days, and more & more folks are exploring AI generated art and putting monies instead to those type of endeavours currently rather than 3D. plus, not too sure that sales for Poser13 have been as successful as they might've hoped, and it is Bondware's, Rendo's parent company, flagship 3D software, and paying developers prolly isn't cheap, or cheap enough.  i'm willing to bet there are other reasons beyond just those, though...

it does seem a rather steep increase instead of the typical business model of gradual increases over time.

keep in mind that the site has been gearing more of its sales mechanisms & purchase benefits towards those who carry Prime on their account.