Why are intravenous solutions generally isotonic and not hypotonic or hypertonic what is the significance of normal saline?

Because it can induce osmosis.

The best solution for intravenous in most condition is isotonic because it will not upset the oncotic pressure of the blood. If you use hypertonic or hypotonic solution, the oncotic pressure can be altered and causing osmosis happen to the cell. The osmosis can cause injury to the cell as its water content will be changed.