The physical issue you are interested in is the Zeeman effect lifting the degeneration of energy states. For that you need an unpaired electron that is tightly bound in a system.
A free electron is... free. You cannot see energy state splitting if there are no quantized energies. (See free electron Laser for something similar that happens if the electrons are free).
In a metal, the situation is again similar. The electrons of the conduction band are delocalized states of the whole crystal. Again, you have an unbound system that would react to the external magnetic field which thereby does not show the effect you're after.
Most importantly, ESR is an easily conductible experiment to which the theory is calculable with a reasonable ammount of work. You see the two important quantum mechanical concepts of angular momentum addition and degenerate perturbation theory at work.
Let me stress again that the important thing is that you can ask a student to actually calculate an expection of the Landé factor and energy splitting that will then show up in the experiment.