finally a very good thread!!!! this is what really makes Interference awesome.
the main reason for which Pop didn't sell very well is wrong marketing, lack of promotion and of communication.
you have to understand that POP was the first U2 album really PLANNED since AB. Zooropa was an offspring of AB, Passengers was an experimental thing with Eno.
so, it was a 5-yr plus break between the 2. in those 5 yrs, things had changed a lot in the biz. grunge had come and gone; new punk had come and gone; REM became the biggest band in the world (from 92 to 95), and so on.
clearly U2 thought that releasing the record and going on tour was enough to sell the album and the tour, but of course it wasn't so. in fact, when they found out the album and the tour weren't selling, they desperatly tried to push it (played an award TV show in Germany in August, played the 2 MTV awards, the Sarajevo gig went live on radio etc...). but it didn't work.
of course, another big problem was the lack of singles. the most radio-friendly track was Staring, which had moderate success, but the following singles (Last Night, Please, If God, Mofo) just BOMBED.
another factor was the media PR. all the word's press came to the first show in Vegas, it sucked, all the press wrote it; some shows sold badly, all the press jumped on it; so the whole era was surrounded by very bad press.
so, of course, the next time around they planned things very well. they started PROMOTING the album in september 2000 (Top Of The Pops on the Dublin roof) and stopped in february 2002 (Grammy-Suberbowl). in between, there was an helluva of things: promo shows, TV awards, the tour, NBA, TV shows, DVD, etc... it was an 18 month plan which worked wonders. as Bono said "the secret is to last 2 Xmas's". it put U2 back on track as a BIG BAND, especially in North Am (the biggest market in the world). they're still cashing on it now, 4 yrs on.
of course, all of that has nothing to do with the artistic side of it. in fact, I think Pop is much better that its follower...we're talking bucks here!!!!!!!