This is a commonly-asked question of many storage vendors, and there are always plenty of reasons. The VMAX uses SMART values returned from the drives, along with its own internal metrics, to make the determination to spare a drive.
If your VMAX is connected to ESRS, the array will phone home and alert support that drives are failing and will need to be replenished.
There is always a chance that the firmware on the drives could be indicating SMART failures ahead of threshold, hence, the VMAX is taking them out of operation too soon. If you feel this is the case, open an SR with EMC or your support provider to analyze the situation. A firmware update for your drives might be necessary.