I think it's similar reasons for the oddness in labeling HDD sizes at odds with how memory is labeled: widescreen lets manufacturers advertise higher diagonals without increasing screen area. Throw in a bunch of influential purchasers who buy based on spec / price tradeoff, and mix in desktop monitor trends (16:10->16:9) following flatscreen TV trends for economies of scale reasons, and you get a very hard to turn around trend.

If it were simple market forces at work, I would have found it easier to avoid buying widescreen monitors and laptops. I couldn't; there wasn't an alternative choice.