As electrostatic force is a conservative force, but friction, occurring due to electrostatic forces is non-conservative.
A wider question would be: why is any force non-conservative if it is a consequence of electrostatic or gravitational forces (not talking about nuclear forces)?
What makes a force non-conservative in mechanics is that it causes transfer of mechanical energy (kinetic and/or potential) into internal energy/heat. In this it is essential that the heated object is treated as a point-like or a rigid collection of point-like objects, so we do not have access to its internal structure. Once we extend our mechanical considerations to account for the movement of the molecules, we treat their energy as mechanical, and the notion of internal energy is no longer needed.
