After all a Kraken can take on most whales, so a food supply has never been a problem. It is also pretty clear that it takes it time devouring any large carcass and essentially finishes it. Yet the population could be comparatively small. This aspect applies to most if not almost all carnivores lacking a convenient herd to follow. The game will often simply not be there.
I do want to add one thing. There is no reason to think this creature as extinct. It is not. However all shipping makes plenty of noise which must easily scare of any Kraken. Until we replace our present power regime we will see nothing.