See, the whole “They had bigger Christological fish to fry” thing, occurred to me.
Yet, it seems that the first occurrences of writing on the Assumption actually come during the height of Christological controversies. Which makes sense, because Marian doctrines were sometimes at the crux of resolving and instigating Christological disputes (Theotokos, Nestorius, etc.). So, I am unsure.
Besides, this kind of thought, (It wasn’t disputed; not the biggest issue; etc) it might miss the point.
Before this would be a doctrine, something brought up to be fought about or affirmed or anything, it would be a historical event. It would be a pretty noteworthy historical event, no? The kind of thing someone would mention, right?
BUT, maybe the Johanine community was less into evangelization (as some suggest), and maybe given its internal Christological fights we see play out in the epistles, they didn’t further complicate it by talking about the Assumption. So, the “bigger fish to fry” argument probably does provide a partial answer.
And I suppose that the persecutions did keep a lot from getting written down. The hagiographies we have of most people do finally get written a while after their deaths. This is maybe the best explanation; stories of saints’ deaths tend to go a long while before being recorded, even in the case of people we know knew them (Athanasius knew Antony; Gregory knew Macrina, but it was years before either of them wrote anything, and of course, they were literate, making them exceptions in antiquity, so “why need any writing” you may have a point).
And then, hagiography doesn’t really become a genre until the persecutions subside, so I suppose the “Persecutions keep people from writing” argument has a good point.
The partial answers probably all come together to give a good one. If anyone else has any thoughts, please share!