why cant we use B or E front disc?

You have to change to '73 up spindles. The drum spindles have a different diameter spindle, so, the bearings are different.

Technically, once you switch to '73 up spindles, you could be using B or E body disks. '73 up E-body spindles are the same as '73 up A body spindles. Same rotors.

Regardless, unless you use a conversion kit (AR engineering, Scarebird etc), you have to change the spindles. Which also requires either changing to big ball joint upper control arms, or using the tapered adaptors that Dr. Diff sells to adapt the smaller ball joint to the later spindle, which has a larger taper for the larger ball joint.