Why stepped write on DVD rather than progressive like CD?

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).