Why in a conductor charge is transfer by free electronsrather than by positive charges?

What particles are you comparing them with? Basically, among the common particles, the positive charge would be a proton. Compared to electrons:

  • Protons are almost 2000 times more massive, and can therefore not move around freely.
  • They are bound to their atoms by strong nuclear forces.

Positive ions (entire atoms with missing electrons) can still move around, and in some kinds of current it is actually positive ions that transfer the charges. But in a solid, they can't move around freely.