Because it's a weak setup. 7.5" ring gear, 1.17" shaft diameter, 27 spline.
The Super35 will bump you to 30 spline, 1.31" shafts, better but still weak. A 35" tire is a lot of weight to be throwing around on a shaft of that size. I'm not sure I would even trust that set up on the street, unless you are underpowered.
8.8" vs. 35 Ring Gears
35 vs. 8.8" Shafts and Spiders
Now, don't take this the wrong way 'cuz we're here to help ya, but do you really need to be running 35s and lockers if you're asking questions of this nature? That's a pretty big setup to be running if you're just getting into this. You're opening a huge can of worms to step up to this height, believe me I've been there. Don't try to go too big too fast, you'll regret it.