Why did this comet disintegrate when its sibling was able to hold it together?

On December 28, 2019, a new comet was discovered by ATLAS (the Asteroid Terrestrial-impact Last Alert System). Called C/2019 Y4 (ATLAS) — let's call it 2019 Y4 for short — it had an orbit that brought it to within about 38 million kilometers of the Sun, which is inside Mercury's orbit. Pretty close to the Sun.

In February 2020 the comet brightened rapidly, raising hopes it might become naked eye visible, but about a month later it became clear the opposite was going to happen: 2019 Y4 was disin...