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I'm trying to understand why the code calls out the need for a second ground wire run
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It doesn't say that two grounding wires are needed. It simply says both the mast and the grounding block must be grounded. A reasonable interpretation of the code's lack of detail would be that a continuous daisy chain connection would meet both the letter and the intent of the electrical code. Of course, the AHJ, if any, has the final say and they are not obligated to be reasonable.
NEC 810.21 (I) (2014 version) Specifically states that "Common Ground. A single bonding conductor or grounding electrode conductor shall be permitted for both protective and operating purposes."
BTW, do not confuse the antenna system's grounding for a lightning protection system, that's a different animal completely that has its own set of rules.