It will be a long time before the full circumstances of this accident will be made public by the relevant State authorities. That said, reports appear to confirm the crew pressed on below the MDA in poor forward visibility caused by heavy rain showers. The circumstances were similar to the Lion Air crash into the water on short final at Bali. No doubt there have been countless similar accidents. One of which was at Kai Tak Airport Hong Kong on 31 August 1988 where CAAC Flight 301, a Hawker Siddeley Trident, crashed on short final for Runway 31 while landing in poor visibility. See:
https://en.wikipedia.org/wiki/CAAC_Flight_301
That report stated in summary that the cause of the accident was a combination of pilot error and bad visibility; A cause common to so many similar accidents.
In the 1950's the USAF Instrument Flying School conducted a series of trials using a North American Sabreliner See:
https://en.wikipedia.org/wiki/North_American_Sabreliner . The purpose was to observe the pilots conduct in very low visibility approach and landings manually flown. It was found that in many earlier accidents involving poor forward visibility, both pilots went heads up approaching the DH and because both were staring through the windscreen trying to spot the runway, they not only failed to detect the aircraft going below the glide slope but also failed to detect a high sink rate caused sometimes by inappropriate power reduction by the PF .
The reason for both pilots going heads up seeking visual cues at low altitude was put down to a survival instinct syndrome. It takes discipline for the PM to stay heads down while the PF is looking ahead hoping to see enough to land safely. The PF is subject to visual illusions especially if heavy rain impinges on the windscreen and a false horizon is seen but not realised until too late. If the PM tries to sneak a glance up through the windscreen in an attempt to spot the runway when he should have the discipline to stay heads down to catch any unwanted descent below a glide slope or inadvertent increase in sink rate, the chances of both pilots being subject to a false horizon is very high. It becomes a case of who is minding the shop, so to speak
Many years ago, the former Ansett Airlines had a SOP where the PM was required to make a call at 500 feet on all approaches (visual or IMC) of the current height, airspeed and VSI (sink rate) reading. Hence a typical call could be: "500 feet - Bug plus five - Sink 800" Of course things could go awry below 500 feet and hopefully one of the two pilots would pick this up. But what that call did was to force the PM to go heads down to read the current state of the flight instruments. The PF would also be aware of his own instrument indications of course.
Having extensive experience at landing in heavy rain at several Pacific atolls, and aware of the insidious effect caused by false horizons in heavy rain, this writer had a personal SOP when approaching to land in heavy rain or poor forward visibility and that was a request for the PM to give an additional call-out at 200 feet agl of height, speed and sink rate. That was because by then as well as flying on instruments, I would be glancing up peering ahead looking for visual cues possibly through a rain affected windscreen and thus be subject to visual illusions. If the PM called an out of tolerance reading of his instruments at 200 feet, there would be enough reaction time and energy to make a go-around. While admittedly pure speculation, the precaution of a 200 feet agl call by the PM may have gone some way to preventing the Air Nuigini accident at Truk. As it was, its a good bet that both pilots were heads up looking for the runway in blinding rain at the time of impact with the water.