A Simple Intensity Modulation Encoding is rejected because it does not provide synchronization; a string of 1s or 0s would have no transitions
The 4B/5B code is chosen over Manchester code because it is more efficient (80% vs. 50%)
The 4B/5B code is further encoded using NRZI so that the resulting differential encoding will improve reception reliability
NRZI (NonReturn To Zero Inverted)
1: A transition at the beginning of the bit interval
0: No transition at the beginning of the bit interval
The specific codes chosen for the encoding of the 16 four-bit data patterns guarantee no more than three zeros in a row. This provides for adequate synchronization
