memory on graphics cards is quite a bit faster than "regular" memory.
i'm no expert, but i can't recall ever reading a review of a memory card on which the memory chips were Micron. Samsung, Infineon are the brands i recall i saw quite frequently.
possibly Micron don't specialize in that kind of memory.
One Ring to rule them all, One Ring to find them,
One Ring to bring them all and in the darkness bind them
In the land of Mordor, where the Shadows lie
M.C. Escher - Reptilien