Why is there no alarm when the transponder is turned off? When the transponder on Malaysia Airlines Flight 370 went off, there was no ALARM. The satellite kept getting pings, and apparently "knew" that the transponder was off. Would it not be a great idea if the Malaysia Airlines got an alarm indication that the transponder was off on one of their planes? The airline could have radioed the pilot and asked what the trouble was. If the pilots did not respond, some jets could have been scrambled to checkout what is going on. The scrambled jets could have followed the airliner for hours and watched them crash in the ocean, when they ran out of fuel. We would know EXACTLY where the airline crashed!
Because it's not worth it.
Quick, name me another accident where a transponder-off alarm would have been exactly what was necessary to save the crew, the passengers, or the airframe?
Hint: there aren't many.
The number of false positives would drastically outweigh the number of actual events where a transponder alarm would have been able to do some good.  For example, air traffic control might ask an aircraft to turn off their transponder.  Nothing dangerous there, but alarms would be going off back at dispatch.  Good idea on paper, but not practical enough to make it worthwhile in the real world.