Trump makes news. If one news outlet doesn't cover him others will and the one that doesn't will lose out.
Ted Cruz doesn't make news -- or at least his campaign doesn't -- so there's no pressure (or much reason to cover) him.
I'm not sure what you mean by how they "treat" Cruz or Trump, though. The election is over a year away. It's far from certain who the nominees will be, and (apart from the random bloggers that get posted here) the mainstream media hasn't even begun to fight.
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BS, top to bottom.