I’m not a rocket scientist but it seems to me that they should place a transponder in these black boxes... you put the transponder on a low voltage standby until it receives the challenge pulses then it “wakes up” and it can begin a full power transmit of its response...
I have wondered this myself. A friend of mine (former senior 747 pilot for United) told me when I asked that he could think up two reasons: 1) cost, and 2) FAA foot dragging.
I have used transponders in research for decades. They are somewhat more expensive, but they will last for years (we have had them last for 3-4 years, depending on the amount of use.
Also 37.5 kHz (which I believe is the pinger frequency) is fairly short range in the ocean. Normally, frequencies used in deep-ocean work are 12-13 kHz or less.
Of course, there is this question: Is the Malaysian airline paying for the search?/s