This is a misunderstanding. Firstly, Christopher published tons of material after the Silmarillion: the whole History of Middle Earth series, plus Unfinished Tales, Children of Húrin, Beren and Lúthien, The fall of Gondolin, and some other books.

But more importantly, JRR Tolkien intended to publish the Silmarillion as an actual book with a canon narrative, just like Lord of the Rings, but he died before doing so. The Silmarillion, as published, was the product of an earlier thinking of Christopher Tolkien that tried to reconcile different versions of unfinished materials, written in a large timespan (a lot of it even before Lord of the Rings), into a single coherent piece. That is, to do the job of his father. At some places, Christopher had to write stuff almost from scratch, to fill gaps that his father intended to write but never managed to. And he did all this in just 4 years, with the help of just one other dude. So Christopher wasn't just the editor of the Silmarillion, he was also its uncredited co-author.

Christopher later thought that presenting The Silmarillion as the authoritative work containing a single narrative, instead of showcasing the multiple, contradictory narratives that Tolkien wrote over the years, was too reducing and misleading. If Tolkien had managed to publish the book, he would have blessed a single coherent narrative as canon, but he didn't.

As such, Christopher decided that his next work, The History of Middle Earth series (HoME for short), would actually showcase all draft versions and not just the one that Christopher decided to be canon. In this sense, HoME works better as the worldbuilding notes than the Silmarillion (and also other books like Beren and Lúthien, that describe many drafts of the story rather than just the one in Silmarillion)

Here's what Christopher has to say about this, https://en.wikipedia.org/wiki/The_Silmarillion and https://en.wikipedia.org/wiki/The_Silmarillion#Posthumous_pu...

> Because J. R. R. Tolkien died leaving his legendarium unedited, Christopher Tolkien selected and edited materials to tell the story from start to end. In a few cases, this meant that he had to devise completely new material, within the tenor of his father's thought, to resolve gaps and inconsistencies in the narrative,[3] particularly Chapter 22, "Of the Ruin of Doriath".[4]

> (...)

> As explained in The History of Middle-earth, Christopher Tolkien drew upon numerous sources, relying on post-Lord of the Rings works where possible, ultimately reaching as far back as the 1917 Book of Lost Tales to fill in portions of the narrative that his father had planned to write but never addressed. In one later chapter of Quenta Silmarillion, "Of the Ruin of Doriath", untouched since the early 1930s, he had to construct a narrative practically from scratch.[T 21] Christopher Tolkien commented that, had he taken more time and had access to all the texts, he might have produced a substantially different work. In his foreword to The Book of Lost Tales 1 in 1983, he wrote that[T 22]

> > by its posthumous publication nearly a quarter of a century later the natural order of presentation of the whole 'Matter of Middle-earth' was inverted; and it is certainly debatable whether it was wise to publish in 1977 a version of the primary 'legendarium' standing on its own and claiming, as it were, to be self-explanatory. The published work has no 'framework', no suggestion of what it is and how (within the imagined world) it came to be. This I now think to have been an error.[T 22]

> If there was any other unpublished material worth publishing, his son would have already done it.

I think this underestimates the huge job of publishing Tolkien's manuscripts. HoME was published from 1983 to 1996 and spans 12 volumes and some 5400 pages, and still doesn't contain everything!