Probabley because a B747 operater at M.86 doesnt want to see only 208 knots on his clock at FL500, and a B737 driver doesnt want to see only 178 knots at M.75 at the same level either.
I must admit its rediculous all these new types of long-haul aircraft are still dribbling around at subsonic speeds. But you can blame the politicians, greenies, and traveling public for that.