it takes time (~2 years) for silicon designers to go from idea to taping out silicon. The P550's cores are advertised as having good area efficiency, so it could be both getting rid of the vector extensions to optimize area and they just couldn't incorporate them into the design.

I is the base integer instructions

M is integer multiplication

A is atomic

F is single precision float

D is double precision

G is shorthand for all of the above + 2 others that I honestly have no idea what they do