DDR2-SDRAM Termination methods: passive vs. active
The #1 reason is for lower power consumption.  With passive termination, on really wide data lines, you can have AMPS worth of power going though the termination resistors-- and all of this is just wasted power.

Power consumption with active termination is harder to calculate, because it depends on the data pattern.  But it almost always it consumes less power than passive termination.  In the best case scenario (with half of the data bus high, the other half low) the regulator doesn't provide any current at all.  There is still power being consumed, but it just doesn't come from the regulator.  So there is no "wasted power" in the best case.  

Active termination can also be smaller.  There are less resistors around your space-constrained chips.  The regulator takes up space, but you have more flexibility on where to put it on the PCB.  I don't remember the numbers, but passive termination might require higher wattage, a.k.a. bigger, resistors due to the power consumption.