The question is justified, I'll try to answer it from my point of view:
While it was Sadgit's idea originally (and he was the one who jumpstarted all this), I would also have chosen to convert "The Shadow Rune".
And although "The Shadow Rune" is undoubtedly one of the "worst" campaigns for classic Descent (in comparison), this doesn't really make it a bad source to port over. Most of its fault lie in poor balancing because of its open quest structure (which means the hero powerlevel at which each given quest can be played varies dramatically).
This however isn't as much of a problem for a coop experience, as there is no "other side" to balance the game around. We also can and have to change some things that are hard to convert, but we aim to keep the story and the narrative. And this is one of the things that the "newer" players (those who bought the "Heirs of Blood" release) are missing out on, because there sadly is no way to get the shadow rune today.
While I fully agree that "Heirs of Blood" is the better campaign, "The Shadow Rune" has less complex encounters, which makes it a better pick to convert. At least as the first project.
Also, at the time when "Heirs of Blood" was released as a campaign book, there were plans to also re-release a "Shadows Rune" campaign book.
But then the Star Wars-focus thing happened, the Asmodee thing happened, the we-can-sell-miniature-games-now thing happened and everybody knows the current priorities of FFG in regards to Descent, altough they don't communicate them.
In essence nostalgia might be a big part of the reason why we picked "The Shadow Rune".
And back in 2017 my foolish self believed, that FFG at some point might realize the potential of such a project and create something similar for Road to Legend. But I guess there isn't anyone left that cares (or has the freedom to care).

Not to leave the wrong impression, it's not my conversion alone. I'm just a bit more active posting news, since the project was on halt for a long time.
Essentially it's all thanks to Bruce (who develops Valkyrie) that this is even possible. And some of the content was done by Sadgit, Dan and very recently Charles who already created 3 complete quests on his own and is working on the 4th (which might not sound like much, but creating a quality quest is quite time consuming).
But I personally was a big fan of coop Descent since the release of Redjaks automated overlord and the coop PODs (because I never found players who would want to play a regular campaign due to its antagonistic nature), and when I first heard about it I was convinced that Road to Legend would have near limitless potential. But even thought the first release wasn't what I had hoped it would be, FFG quickly added new content and even added new play modes. Sadly supporting RTL seems to have hit it's peak just before FFG could really capitalize on what they had created. And even though there might still be something in the pipeline for Road to Legend in the future, support for Descent has essentially stopped. I don't want to sound ungrateful though, because most of the content for RTL is free, and FFG could easily have charged money for it.
But I digress, and to answer your question: Expansion content for Valkyrie might happen in the future, but it would be important to increase the size of our team for that. Since we currently only have 2 active contributors it will take some time to finish the Shadow rune. Finishing the Shadow rune is not an unrealistic goal though, and I think will make it at some point. What happens after that is in part up to player interest, and which features Valkyrie can provide in the future.