The AM antenna is of such construction as described by n5tp because of the much lower band of frequencies for which it needs to be receptive. AM broadcast is 535KHz-1605KHz, compared to the FM band at 88MHz-108MHz. Now look at the formula for resonant frequency of an LC (inductve-capacitive) circuit, in which the AM antenna coil of wire acts as L:
ƒo = (2×pi×/LC)-1
That's supposed to be the square root of LC but the symbol font seems to be hosed up for some reason. You can see that the values of L and C must be larger at lower frequencies. A coil of wire wrapped around a ferrite core has much greater inductance than a straight telescoping rod, and therefore works better at lower frequencies.