Originally Posted by Slaninar

1) Because it is achievable with a cheaper, 3x8 drivetrain. Same range, with same spacing (if not even tighter).
2) Because most differences are at the slower part of the cassette, where 2, or even 3 teeth jumps are often even better and more convenient. 13-14-15 is where the 2 teeth jumps are most important. For the rest - not so much.

An, even more obvious example would be an "upgrade" from 10 to 11 speeds. More expensive parts, even more rear wheel dish. For what improvement exactly? I mean, c'mon! Unless you're paid to cycle, you really don't need that one extra gear.