Why was northern Israel carried away to Assyria?
Because the Lord was very angry with Israel for walking in all the sins of Jeroboam and not departing from them. He rejected them, afflicted them, delivered them into the hand of spoilers, and finally removed them out of His sight, so they were carried away to the Assyrians.
Les explains that the northern kingdom, the northern ten tribes, had been rent from the house of David when they made Jeroboam the son of Nebat king. The children of Israel walked in all the sins of Jeroboam and did not depart from them. Therefore the Lord was very angry with Israel and removed them out of His sight. He rejected all the seed of Israel, afflicted them, and delivered them into the hand of spoilers until He had cast them out of His sight. The summary verse says that until the Lord removed Israel out of His sight, so was Israel carried away to the Assyrians. Only the tribe of Judah was left.
For the children of Israel walked in all the sins of Jeroboam which he did; they departed not from them;
Until the LORD removed Israel out of his sight, as he had said by all his servants the prophets. So was Israel carried away out of their own land to Assyria unto this day.