It's not any more demanding than the live tiles. I don't think that's the reason.I was thinking about this these days, maybe the effect is too demanding to render ? The vast majority of windows phones consists of devices with low end gpus, i suppose it would be hard to optimize for all of them, and if microsoft decided to put it only in the few phones with enough power, a lot of people would be pissed.
Maybe it simply doesn't fit with the design language that they're aiming for. Design is an art. If the artist chooses to go a different direction with their piece of art, that's their prerogative.
... But then it becomes pretty bloated and would run like crap, much like Windows Vista. *shudders*
No frosted glass and transparency please - these are Windows 7 style. Windows 10 is flat, solid, tablet like style.
I think the design language they're after now is to take things from iOS and Android and throw them together. Circle profile pictures, wireframe icons, rounded toggles. Frosted glass? Sure, why not?