This is standard!
If you lower a car using only springs, your making your shocks do a lot more work, this results in them lasting a lot less.
OEM shocks are designed to work with OEM springs, change the springs for lower ones your OEM shocks have to work a lot harder and hence the failure.
Generally its a bad idea to change only springs on a car due to such reasons, the other being handling generally as normally shocks and springs are tuned to work together.
It same on Porsche, some guys lower them using just springs, then 2000 miles later, the shocks are ruined.