In addition to the other two excellent answers, there's another reason for the apparently higher issue count:
US-EAST-1 is enormous. It's now made up of 10 data centers with tens of thousands of machines (and associated infrastructure). There are thus more moving parts, and more points of failure. While Amazon builds a bunch of redundancy into its systems, "smaller" issues will tend to impact a larger number of users in US-EAST-1.
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In addition to the other two excellent answers, there's another reason for the apparently higher issue count:
US-EAST-1 is enormous. It's now made up of 10 data centers with tens of thousands of machines (and associated infrastructure). There are thus more moving parts, and more points of failure. While Amazon builds a bunch of redundancy into its systems, "smaller" issues will tend to impact a larger number of users in US-EAST-1.