Because for those who don't have pop-up blockers, you can still make some money. On sites that rely on traffic from search engines and have good content, the entry pop up isnt going to stop most users from still viewing the page. A lot of people these days really don't have pop up blockers, and the revenue producing visitors are usually using outdated hardware and software anyway
SP2 IE6 included a pop-up blocker, many people with pirated versions of windows do not have SP2. Also, most revenue producing visitors do not use mozilla or opera. I do agree though that the majority of users now days do have a pop-up blocker, I should have said that most revenue producing visitors do not have a pop-up blocker and that there is still a large amount of users without pop-up blockers.
Still the number of guys without a pop up blocker are greater than people with them. They are still good options for site, which are high traffic, but kinda one use stuff. By that I mean, that people do not visit them everyday, but whenever they need to use that service. And I agree Tyler, popups piss me off too, but if a site does offer exemplary service, then I am willing to face it