‘Then these men thronged to the king, and said to the king, “Know, O king, that it is a law of the Medes and the Persians, that no interdict or decree that the king establishes may be changed.” '
The men were relentless in their pursuit of Daniel. They knew that they had got their way. They stressed to the king the unchangeableness of the law. In a way it was a good law. It prevented the law being changed suddenly to suit someone's convenience. The same applies in many civilised societies today, in that the law cannot be changed retrospectively, although modern law courts are not quite so relentless. He had no choice. He must carry out the decree.
The choice lay before him, seal Daniel's fate or be reported to Cyrus about his failure to fulfil his own decree. The consequences of that would not be pleasant for him, and it was very likely that the overlord would enforce the decree anyway to maintain the sanctity of the law. So he gave way, partly no doubt because he did recognise the binding nature of the decree. He had been caught out, but he was not at all pleased.