As a retired airline pilot with 15,000 hours in a 747, I am quite sure that the ELTs you are talking about today on MH370 are actually more equivalent to EPIRBs. Little airplanes have ELTs that activate on impact. Airliners have ELTs that are activated by salt water and they are located in the rafts and cockpit equipment. To my knowledge no airliner has an ELT that mounted on the airframe and activated on impact like the ones on small airplanes. Thus there is no reason that they would have been turned on unless a slide/raft inflated somehow and water got to the ELT, and therefore no reason their signal should have been picked up.