Just found a Q&A in the NYT about this exact question. Here you go:
Q. How could a Boeing 777 simply vanish? Aren’t they always tracked by radar or satellites?
A. Radar coverage is not universal, especially over water. In areas without radar, pilots are generally required to radio in their positions at fixed intervals, mostly to assure that air traffic controllers can keep aircraft out of one another’s way. Between intervals, something could go wrong.
Planes like the 777 also have automatic systems that send out data on engine performance and other technical functions. Those signals go to a maintenance base, not to air traffic control. Air France used those signals to help determine what happened when its Flight 447 disappeared over the equatorial Atlantic. Investigators may be doing something similar in Kuala Lumpur.