That device will last for three and a half hours on a 700maH battery. The devices here will last for months, if not years, on a single coin cell. Something like a wireless moisture sensor so you can more efficiently water your garden doesn't need 8 megs of ram. In such a case, why have waste all that power on parts of the chip that are never going to be used? There are a lot of areas where a small chip can do the job just as well, if not better, than a much larger chip using a more conventional OS.

8MB of RAM alone probably consumes order(s) of magnitude more power than typical low-power microcontroller Zephyr is targeting.

Also have you seen the size of todays microcontrollers? They are absolutely tiny:

http://imgur.com/a/RsPjw

The less memory, the less chance of a memory error.

Less memory means less code, which means fewer bugs.

I'm all for efficiency, speed, and fewer bugs but isn't there a point where you are trading actually useful features for size reduction?

Because it's cheaper.