Rebels in Coruscant.

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The simplest answer would be that the higher the tier, the less likely are the rebels to stick out their neck for someone who isn't already deeply affiliated with them. 

Remember that Coruscant is an Orwellian police state; the highest tiers have guards and cameras (almost literally) on every corner and even the middle tiers still have regular patrols. This renders any sort of operations, covert or otherwise, in higher tiers of the city incredibly difficult and risky. In other words, if you get in trouble with the Empire in the higher tiers of Coruscant and aren't already a member of the resistance movement, you're essentially on your own. 

That said, if your character can pull off getting to the lower tiers (though that's no easy prospect), the rebels might be willing to shelter you from the authorities. After all, aiding you poses much less risk to them in the parts of the city where the Empire's control is laxer.