Unknown. However, there is a very good candidate.
Possibly the presence of Kyber crystals
As we know from Ultimate Star Wars, the Death Star used a Kyber crystal in its superlaser. The protagonists of Rebels have seen other signs of Death Star construction.
The Rebels on Lothal sought to prevent a Kyber crystal shipment (probably from Lothal) from reaching the Empire.
From "Breaking Ranks" (Star Wars: Rebels 1x04):
HERA: What's the alternative? Do you want to stop that Kyber shipment or not? You know what the Empire could do with that crystal.
KANAN: Nothing good.
HERA: And we'll only get one shot at this.
The Kyber crystal resonates with the Force.
This suggests that Kyber crystals are mined on Lothal, which indicates that they may be supplying them as material for the Death Star.