Why did Israel's borders change so much?

Answer 1

Israel and her neighbors have been subject to migrations, war and conquest, thus affecting her populations, ruling dynasties and borders. Israel (or large parts of it) has been under the control of the:

Canaanite tribes, ancient Egyptians, Israelites, Assyrians, Babylonians, Persians, Greek Seleucids, Hasmonean Jewish kingdom, Romans, Byzantine Christians, Muslims, Seljuk Turks, Egyptian Fatimids, Catholic Crusaders, Mamelukes, Ottoman Empire, Napoleon's French army, and the British.

See also:

Jewish history timeline

Answer 2

If you are referring to the historic territory of the Land of Israel, please see the answer from my colleague Dan Galilee below.

If you are referring to the Modern State of Israel, the reason the borders change so often is that Israel's neighbors consistently felt the penchant to go to war with it (1948, 1956*, 1967, 1967-1970, 1973, 1981, etc.) and attempt to push its inhabitants into the sea. However, they often came with more Propaganda than strategy, resulting in Israeli gains in defensive struggle.

*The 1956 War was initiated by Israel after Egypt nationalized the Suez Canal in contravention of international treaties, but not in direct opposition to Israel.