Complex sentence: This is because AM3 processors will have a new memory controller supporting both DDR2 and DDR3 RAM on them, allowing backwards compatibility with AM2 motherboards, but since AM2 processors lack the new memory controller, they will not work on AM3 motherboards.
Split into simpler sentences: This is because AM3 processors will have a new memory controller supporting both DDR2 and DDR3 RAM on them, allowing backwards compatibility with AM2 motherboards. Since AM2 processors lack the new memory controller, they will not work on AM3 motherboards.