Because infrared doesn't work very well in water, and crashes in water are the biggest challenge for finding wreckage.
Airliners also carry emergency locator transmitters†(ELTs), which, when activated by a crash, transmit radio signals which can be triangulated via satellite. However, ELTs do not work underwater. No ELT signals from MH370 were heard.
Airliners also carry emergency locator transmitters†(ELTs), which, when activated by a crash, transmit radio signals which can be triangulated via satellite. However, ELTs do not work underwater. No ELT signals from MH370 were heard.
†http://en.wikipedia.org/wiki/Emergency_Locator_Transmitter