Each Shard has an intent. Many of these intents contradict each other, sometimes prompting battles between Shards. What was Adonalsium's intent? It might have had no intent, since the intents would have canceled each other out, or it might have had all 16 intents at once. This would cause Adonalsium to always be in conflict with itself. Given the strength of the intents of the Shards that we have seen, it would also be in extreme agony, and never actually be able to do anything. A solution to that situation would be for Adonalsium to shatter itself, hence my theory.