One significant reason that China was isolated from influence by other civilizations was because of what

The most significant reason is the geography.

China has been isolated by the other civilizations from the Middle East and Europe mainly because of the geography. In between China and the other civilizations there were the highest mountains in the world, the Himalayas, big dangerous deserts and multiple arid steppes. So the Chinese did not saw a reason to risk and go westwards since they already had everything they needed at their own territory.