why coaxial cable not used in oscilloscope?
Scope signals capable of 30MHz usually have controlled impedances.

Rule#1


  It is impossible to have signal integrity when the rise time is faster than the propagation delay of the signal unless you have controlled matched impedances. Otherwise you automatically get ringing. 


Ground lead inductance is a major cause of ringing even in coaxial probes that must be avoided for measurements>30MHz or Tr&lt;10ns by removing tip &amp; gnd clip and only using tip/barrel ring.

e.g. stripline, microstrip or braided coax. or in high end scopes only semi-rigid coax. are used with SMA or similar RF connectors.