Why were these crash location not located via ELT? On Jan 14th 2019, at 9.10 Indonesia time, the CVR of the crashed Lion Air JT-610 has been found. Another vital device was found earlier.

There are three modern airplanes crashed in to water in Indonesia:


Adam Air, registration PK-KKW, Boeing 737-400, flight KI514, on 1-January-2007, crashed into Majene Sea due to the failure with the Internal Reference System (IRS), rested in 2,000m below the sea surface.
Air Asia, registration PK-AXC, Airbus A320-216, flight QZ8501, crashed on 28-December-2015, into Karimunjawa Sea, due to the failure with the rudder-travel-limiter.
Lion Air, registration PK-LQP, Boeing 737 Max-8, flight JT-610, crashed on 28-November-2018, into Java Sea. The crash cause is still under investigation.


Those three crash locations were found not by the electronic instrument system, but by fishermen.

Then my question is: why did the modern airplane has not been located via its ELT?
ELT's are not required to be built to the same standards as the CVR and FDR. That means they are much more likely to be destroyed in a severe crash. Also, ELT's (the old style 121.5 and 243 mhz and the new 406 mhz) are not designed to function under water or mud.