Why do DVD's write at 4x then 6x and then 8x instead of gradually getting faster like CD writers do?
Look up CLV, Z-CLV, P-CAV and CAV writing strategies.
At lower rotational and/or data rate speeds, CLV is much faster.
Most 12x and 16x writers will most likely use P-CAV/CAV.
My guess is that Z-CLV is easier to implement (writing strategy etc).