Why was Daniel thrown into the lions' den?

Answer: He prayed to his God

Despite the king's decree, Daniel kept praying to God three times a day, and his enemies reported him to the king.

“Now when Daniel learned that the document had been signed, he went into his house, where the windows of his upper room opened toward Jerusalem, and three times a day he got down on his knees, prayed, and gave thanks to his God, just as he had done before.”— Daniel 6:10 (BSB)