Titanic was in the middle between London and NYC, relatively easily within reach of either by using longer waves. Titanic actively sent distress signals.

MH-370 has flown into the uninhabited area of Southern Indian ocean, far away from any places where a shorter-wave signal would be easy to receive, due to the curvature of Earth. MH-370 apparently did not send any distress signals; if it did, these would likely be received and relayed.