Spectral properties of EFM encoding
As already pointed out by Marcus in another answer, you'd want to get rid of the DC component of a signal so you can use AC-coupled stages on your signal processing chain. This would be possible by using a simpler encoding. 

The additional advantage of having low power density function (PDF) at low frequencies is that this makes the AC-coupling easier, since you can use "smaller caps" (i.e. caps with smaller capacitance) and consequently amplifiers with smaller input impedance (to avoid attenuation, the input impedance should be much higher than the impedance seen "looking back" at the preceding stage, and this latter includes the impedance of the coupling cap).

A signal with no DC level, but with high PDF at low frequencies would require bigger coupling caps to avoid signal integrity issues. To be more specific: lower capacity caps would lead to a lower cut-off frequency that is higher, hence leading to more signal distortion because the frequency response of the system would attenuate more in the frequency range where the energy content of the signal is more significant.