From linked xkcd what if: Gravity in low Earth orbit is almost as strong as gravity on the surface. The Space Station hasn't escaped Earth's gravity at all; it's experiencing about 90% the pull that we feel on the surface.
The majority of energy expended in getting to orbit is spent in getting to orbital velocity (going sideways), rather than getting to orbital altitude (going up). Balloons only help in going up, and only to where the density of the atmosphere is so low that the balloon is no longer lifting, they are still far short of the altitude necessary for orbit. Balloons top out at around 20-30 miles while LEO is ~100 miles.
So launching using a balloon only really gives you a very small fuel savings
That's the issue though. The hard part about achieving orbit isn't the height, it's the speed. In both a ground launch and a balloon launch you are starting at 0.
This article does a better job explaining it than just about anything else on the web:
https://what-if.xkcd.com/58/
So complex schemes to lift the rocket to the edge of space don't end up buying you very much.
