Why npn transistors are preferred in water level detector circuit?

In this circuit all the transistors are working as a switch.

When the water touches the metal contact in which base of each transistor is connected, a small current flows and turns on the transistor.

When a transistor turns on, LED connected to it glows.

Thus LEDs will be turned on depending up on the level of water. By using a relay we can also control the level of water.

-suuh