Why not 256?

Why isn't the height limit for mcpe 256? Or its going to be implemented soon?

I do not think so.

Because 256 can be laggy for MCPE.

Usually mobile devices don't have good GPUs and they can't load lots of graphic data, so they will lag with a height limit of 256.
The explanation is here: http://forums.pocketmine.net/threads/limitheight.8064/#post-84592