simple answer, size. most other units will not fit under the stillen hood.
most people don't like the stillen hood already, but imagine if you needed a bigger one?! most other units that flow better or more for more power will require a bigger hood, there's just no room on the Z.....
Other SCs like the vortech etc... fit under the hood better, but you're also not getting the imidiate boost response that the stillen gives, but due to it's design, it can flow more and more efficiently at higher boost levels, thus allowing it to create more power than the stillen.
it's all just a simple trade off on what you're looking for...

 

True, but if they'd use a bigger blower I'm sure people wouldn't have a problem using a cowel hood rather than a hood that looks like a pimple needing popped. (I do like the stillen hood on the G's though..)

 

Said it well.

Stillen is junk, and some expensive junk at that.