Because it’s a protectorate, an extension of US power. It’s close enough to seize the Suez Canal if required, and joins with Gibraltar as the nodes for controlling access to the Mediterranean.
Plus it prevents any kind of pan-Arab state forming that would want to include Egypt, which was considered to be on the cards in the 1970s and 80s, and can be used and is frequently used to bomb US enemies such as Syria and Iran with less risk of igniting a direct confrontation because it’s done by proxy.