Why would the Federation leave a major strategic asset like Corvan II (40% of the Federation's dilithium) completely undefended?

Question

In Star Trek Discovery S1:E4 The Butchers Knife Cares Not for the Lamb's Cry we see the Discovery receiving a distress call from Corvan 2, a mining planet that's under attack by Klingons.

Corvan 2 is a chief dilithium producer for Starfleet, and no other ships are close enough to help, so Lorca decides to use the experimental spore drive of his ship and zip right over.

My question is: Why would the Federation leave a major strategic asset like Corvan II (40% of the Federation's dilithium) completely undefended?

Answer 1

The distress call says that their patrol ships were destroyed. Also note that they have heavy shielding (just not heavy enough to withstand a full scale attack indefinitely):

We're under Klingon attack. Our patrol ships have been destroyed. Our magnetic shields are degrading under bombardment. They will fail in six hours.

And also:

The Klingons ambushed the blockade that was protecting the colony

We don't know what comprised their patrol ships - clearly it was insufficient to deal with a large scale surprise attack, but certainly it doesn't sound like they were completely undefended either.