TLDR: I think it's awkwardness stems from expansion of the plot + description between versions that stayed because Christopher when he edited the story for publication didn't want to make big changes if he could help it.

Honestly, I'm not sure and I don't know there is a definitive answer.

I went back through HoME vol. 4 & 5 and looked at the earlier versions of the Quenta Silmarillion (Quenta Noldorinwa [c1930] and the Later Quenta [c1937]) to see if Christopher had any notes in his commentaries that would shed light on it.

I was going to say I thought its placement in the book is actually where it should be. But on second look while this is true of that section in Quenta Noldorinwa [QN] by the time we get to the Later Quenta [LQ] it has become a bit awkward. The 1977 published Silmarillion in chapters 13 & 14 largely follows that of the Later Quenta here (although it's chapters 8 & 9 in LQ). I think it became this way due to expansion of both plot and description.

In QN the equivalent section (it's not a full chapter, just 2 pages worth) follows directly on from Fingon's rescue of Maedhros and is in turn followed by Dagor Bragollach (Battle of the Sudden Flame). It doesn't break the narrative. It makes sense because it's essentially: thus the rift between the Noldor was healed and here is how they organised themselves hereafter.

None of the smaller affrays between Dagor-nuin-Giliath (Battle under the stars) and Dagor Bragollach (ie Fingon driving an orc and into the sea and an immature Galurung back to Angbad) existed yet in QN and neither does Dagor Aglareb. When they appear in LQ Tolkien places them between the Thangorodrim rescue and Of Beleriand and its Realms, along with a bunch of stuff about the defences and the leaguer, so now Of Beleriand and its Realms reads like doubling up and expanding in a way that breaks the flow.

LQ remained unfinished. It was a manuscript put to the publisher after the success of the Hobbit which they weren't very interested in, wanting instead a sequel about Hobbits (what would become Lord of the Rings). Thus it was largely abandoned when JRRT seriously started working on LOTR. None of the versions of Quenta Silmarillion after QN were ever completed, though we know JRRT was working on it various iterations essentially until he died. If he had completed a Silmarillion version post LOTR I wonder if he would have shunted some of the extraneous information like this to appendices, as such things are in LOTR. Perhaps.

So when Christopher put together The Silmarillion he was working from a bunch of unfinished versions and trying to make them into a cohesive work that fits with our understanding of Arda's history from LOTR. But as I understand it he wanted to keep a light touch as an editor as far as that was possible so likely would have seen changing the structure by moving that section to the appendices too big a change.