Consider the following sentence:

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> In 2013 Tallinn, Estonia, instituted fare-free rides for city residents (becoming the largest city in the world to do so), but car use in Tallinn has only slightly declined: as a 2014 study by the KTH Royal Institute of Technology in Sweden found that car traffic in Tallinn was down less than 3% since the policy was enacted.
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My question is regarding the colon between the words "declined" and "as". Would it be appropriate to put a comma in place of the colon? Take, for example, the following sentence: 

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> I received a cake and many gifts, as it has been my 21st birthday.
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Why would it be inappropriate to put a comma instead of colon in the first sentence? If it is allowed, why would a colon be preferred over the comma?