It peaked at #23 in its first release (end of 1971 into early 1972) and at #53 in 1974, so how much radio play did this ever actually get? I'm guessing it got at least a little the first time around, but maybe not the second time since there are plenty of songs in the bottom half of the Hot 100 that were pretty much ignored by radio.
Since I'd never heard of it, let alone heard it, I got curious enough to look up the lyrics and to skim through the song on You Tube. Based on that, while I know they were trying to get what they considered an important message across, the way they did it just seems like it was cause listeners to change stations with all those voice overs and the sobbing at the end.
On the other hand...the first half of the seventies was a time when a lot of songs about death made it onto the radio. A couple have been mentioned here, but adding to the list would be Terry Jacks with "Seasons in the Sun" (a huge hit at the time), Austin Roberts with "Rocky", Hot Chocolate with "Emma" (so different from their subsequent hits), and David Geddes with "Run Joey Run" and "The Last Game of the Season". So in that sense, "Once You Understand" wasn't completely out of place for its era.
(As an aside, the last time I heard "Seasons in the Sun" on a commercial radio station was probably in late 1986, when 94.9 had recently flipped to rhythmic-leaning CHR/Top 40 as KHYI, Y-95. They were running jockless and commercial free for their first several months, with aggressive ("Get out of the way, wimp!") and sexually suggestive ("Lock in and jerk your knob off") station liners interspersed between the songs. And somehow amidst all that, on one occasion I heard "Seasons in the Sun", albeit from a single that had obviously seen better days as I could hear the scratches. I never heard it again, and I never have understood exactly what the point of playing it was...)