Record companies think 11 is the standard for a rock or pop album, so most artists end up at 10, 11 or 12 tracks; sometimes, production agreements and record deals define a certain (minimum) number too.
In 5-10 years we will see. With iTunes selling single tracks to a much bigger extent than full albums, there´s the return of the single market.
There is no reason why a band couldn´t write 16 tracks per album, I think an album concept (if its not a double album) works fine up to about an hour of playing time. To write and record 16 songs instead of 11 that fit into a concept is more expensive and costs more time because it can be more complicated to do a full fledged album concept with 15 songs instead of 11. I tend to dislike albums which are far too short (less than 40 minutes).
U2 however have been around for so many years, I guess they just got used to 10-13 songs and 40-50 minutes. When you look at new genres/ ie. dance music, you´ll often hear young musicians who grew up with the CD format and who use all the 65 or 70 minutes they got.
