Historically, going back to when the king or queen was an active player in government, I understand historians would say that when the king announced a decision, it would be unbecoming and unseemly for there to be any evidence of dissent or divided opinion among the king's advisers, and it would undermine his authority for there to be made known that there was a division of opinion because then he would have chosen one side over the other.
More recently, we get to a more principled and responsible government. Fundamentally, the government is accountable to Parliament and the government speaks with one voice. The government is accountable on the whole, not just in parts, so cabinet solidarity and cabinet confidentiality are all part of that system that calls upon the government to speak with one voice and to be accountable to the House as a government.