Why is liquefactive necrosis only seen in brain?
Liquefactive necrosis is indeed only seen in the brain in the absence of infection, but it is also seen in the case of some bacterial infections.  The reason is that liquefactive necrosis is, as you said, caused by the release of digestive enzymes and constituents of neutrophils.  Bacteria will, under some circumstances, release those enzymes, causing the cells to be devoured.
The fact that liquefactive necrosis is seen in the brain in the absence of bacterial infection is in part due to the fact that neurons have a higher lysosomal content, leading to a higher tendency toward autolysis, and in part due to a high concentration of neutrophils which swarm into an affected area.

Sources:

What happens after brain tissue dies?, PathologyStudent.com, 2012
Liquefecative necrosis, Medigoo.com, 2016
Necrosis, Cell (Liquefactive, Coagulative, Caseous, Fat, Fibrinoid, and Gangrenous), Rotimi Adigun1; Steve S. Bhimj, 2017