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The selection of high cardinality attributes is crucial for ensuring that load is evenly spread in a DynamoDB table because high cardinality means that there are a large number of unique values for the partition key. In DynamoDB, data is distributed across multiple partitions based on the partition key. If a partition key has low cardinality (few unique values), it can lead to uneven distribution of requests, with some partitions receiving a heavy load while others are underutilized. This imbalance can cause throttling and reduced performance.

By choosing high cardinality attributes as partition keys, the data can be distributed more uniformly across the partitions, allowing for better performance and scalability. High cardinality helps ensure that no single partition becomes a hotspot under heavy load, which is essential for applications that require high availability and responsiveness.

Other options like a fixed partition key, restricted access to data, or focusing solely on a popular device ID can lead to skewed load distribution, which can degrade performance and scalability. A fixed partition key can lead to all the traffic hitting that same partition. Similarly, focusing on just one popular device ID decreases the diversity of partition key values, thus undermining the benefits of partitioning.