Why does this insulator separate two conductors with equal voltages?
It is an insulator so that current does NOT flow through the mechanical link. The mechanical link is not designed to handle high current flows, and may fail if subjected to high current flows.

In a fault condition, you do NOT want to have your mechanical supports fail because they have high current flowing through them.

In non-fault conditions you still don't want to have current flowing through your mechanical supports: they would find an equilibrium where the small fraction of the current flowing through the steel would keep it hot enough to not conduct more current.  The hot steel would be susceptible to corrosion, and the threaded connectors would be susceptible to creep.

Note that the voltage is irrelevent for this mechanical support. The insulator is there because, even at 3KV, trains take a lot of current out of the supply. You can get the same problems with Low Voltage (48V) supply systems.