Because it's incredibly irritating;
Because it interrupts you when you've just started to read;
Because the popup-tastic 90's are a time nobody wants to relive;
Because (in this case), closing the popup actually seemed to reset the scroll location too;
Because it can totally FU browsing on mobile; (but who cares about mobile anyway? We're supporting people from 1997 maaan!)
Because it's obnoxious.
I decided a few weeks ago that I would no longer read any sites, articles or blogs that do this. I am Jack's bounce rate.