Why does a semiconductor have fewer free electrons than a conductor?
you mean the number of free electrons that can drift from atom to atom in conductors and semiconductors, semiconductors have far fewer free electrons that can move (not bound to the atom) freely than conductors. If they are given energy, electrons are able to free themselves from their atom and flow. Silicon and Germanium are well known semiconductors. The electrical property of semiconductor can be changed by adding controlled amount of foreign atoms.
The question itself is a bit misleading, so here's a quick summary.
A good conductor such as copper or silver has a single electron in the valence (outermost) shell. Being alone out there, it is very easy to knock it off of an atom thereby creating a free electron. An intrinsic semiconductor such as silicon or germanium have four electrons in their valence shell. That makes them harder to knock out of their orbit than the conductors but not as hard as the noble gasses which have a full valence. In a sense, they occupy the middle ground for gaining or losing electrons.
What truly makes them useful is when we use doping to put an extra electron in the the crystaline lattice with phosphorus or to remove an electron to make a "hole" in the lattice with boron. These are what give us our extrinsic semiconductors that we use to create transistors, diodes, and other useful electronic components.