Rather long prose versions of the Great Tales
To simplify their names somewhat, these were:
The tale of the Nightingale (Beren and Luthien)
Narn e-DinuvielThe tale of the Children of Hurin (Turin and so on)
Narn e-mbar Hador (includes
Narn i Chin Hurin)
The tale of the Fall of Gondolin (Tuor, Earendil and so on)
Narn e-Dant Gondolin ar Orthad en ElSo these tales have both "short" chapters in
Quenta Silmarillion and long prose versions too. And possibly long versions in poetry (see
The Lays of Beleriand for examples), at least for some of these tales. Basically
Quenta Silmarillion is meant to be a history "in brief" drawn from many older tales.
And the reason we have only the one long prose
Children of Hurin constructed by Christopher Tolkien, is that he felt there was enough of this to work with -- at least in the form as revised by Tolkien after
The Lord of the Rings was "finished" but not published, or as worked on again in the later 1950s.
The long version wasn't finished, especially in some parts, but Christopher Tolkien felt he could construct something "like" what his father might have produced, even though he could not include every latest idea (just like with the constructed Silmarillion), as that would, or might, make it difficult to produce an internally consistent version using what Tolkien had set to paper.
That's simplifying things a bit, but for example, Christopher Tolkien knew that his father intended Turin to be wearing the Helm of Hador when confronting the Dragon at Nargothrond, but he chose an earlier idea instead (the Dwarf-mask) because there wasn't enough actually written down and incorporated into the story to give the fuller history of the Helm as Tolkien imagined it. So both constructed versions (Silmarillion chapter, and the one volume Children of Hurin book) have the Dwarf-mask, even though we know Tolkien had rejected that idea for the Helm of Hador.
And if you have read
Unfinished Tales, for instance the tale of Tuor and his Coming to Gondolin is really Tolkien's revised Fall of Gondolin, long prose version, but sadly Tolkien abandoned it fairly early in the story, and only did some later work on it with respect to Maeglin. For a while Tolkien thought he might have a chance at publishing his Silmarillion along with
The Lord of the Rings in the early 1950s, but when that fell through a lot of the new work (later-ish work) on both
Quenta Silmarillion and the Great Tales (long versions) halted for a time...
... as of course then Tolkien had to get
The Lord of the Rings (and Appendices) ready for actual publication, which took up his time, and he had other life concerns as well obviously. After
The Lord of the Rings was in bookstores JRRT had more time to try and get these things done (although again he had plenty of other things going on in his life too); plus his new tale had done well enough in book sales to give him hope that he might get at least some of his "Silmarillion stuff" published.
But that's another story, and I've simplified things here already!
The History of Middle-Earth is a long series for a reason