They fell out of favor when water signatures for asteroids were a better match, comets had more diversity with higher heavy water ratios. I suspect comets that have spent the last billion(s) years orbiting closer to the sun would have built up layers of ice with less heavy water than comets on highly elliptical long period orbits.
The ice on comets (the more exposed outer layers) may come from the region (distance) of space where the comet eventually came to reside but its original ice (if there was any) remains buried under more recently acquired layers. For example, if Jupiter's gravity flung a water bearing chunk of rock outward onto a long period orbit and that asteroid spent a few billion years gathering up ices further from the sun, measuring that sublimating water/ice wont tell us where the rock originated.
Another study of ~90m lava believed to be from the deep mantle containing 'primordial' water suggested ratios more in line with the sun, so even asteroids may not be the source of our water. The study (and others) argue the evidence shows Earth formed with most of its water. The dust particles that would become the Earth already had water, perhaps water ice helped speed the process of accretion.
