Because Marcus wasn't a T-1000 . T-1000s are composed of mimetic poly-alloy (liquid metal). Marcus was a human who was pretty much converted into a cyborg, with a Big Brother chip installed in the back of his head so Skynet could observe and subtley influence him.
Just a thought, but couldn't there be other post-human cyborgs like Marcus lying around in the ground waiting to be activated?
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You don't need good rear vision because you're always in front!