Why Don't Solos and Warnouns go Epic?

Probably PP has just higher priorities than Solos to release (new casters, totally new models, etc...), but as you have already noticed with the Eyriss case, they actually go Epic, just not often.

In general, I think that the biggest problem is that if PP wants to include a new feature to a faction, for them is usually better release a totally new solo than to make an existing one go epic, putting in place a restriction on list building (since you can't have two versions of the same models toghether).

With casters is easier since you usually don't have more than one caster anyway, so releasing a new one or making an old one go epic isn't that different from the list-building perspective, while more versions of the same solo give a list restriction to the new model.

That said, nothing prevents them to do so when they intend to, as they did with Eyriss.

The named Juniors also all went Epic by becoming actual Warcasters.

It's not necessarily a case of PP not wanting to make epic versions of these lesser characters. Its that by nature of being "lesser", they aren't the main focus. Warcasters and Warlocks are the main characters in the Iron Kingdoms, so they get the lions share of the attention and character development. Hence they go epic more often.

Forgot about her. Good catch. Headslapper too. She's the gal that launched the whole damn company.

Oh yeah! That was an interesting one.

Sales.  Need.  Fits into the big storyline.

The only reason why a character solo or a warnoun would go epic is if it filled a need and could generate enough sales to justify the costs.

Instead of asking why more of them don't go epic, maybe you should look at a particular solo or warnoun that doesn't have an epic version and ask why they would go epic. 

Is there storyline to back up them going epic?  Remember that Privateer Press is very story and RPG-oriented, even if a majority of its sales stem from the wargames that come off of the RPG.  In the grand story there are only so many main characters, mostly the warnouns.  A couple of the warnouns and most of the character solos are not main characters - they are supporting characters that are decorative and serve a function.  For any of these supporting characters to go epic there would have to be a major turn in the story that would require them to step up, do something spectacular, and evolve from that.

What would the design of the epic version be such that it would fill a niche instead of over-lapping someone else's roll? 

If you made an epic version of a character solo or warnoun, would it be compelling enough that it would generate good sales?

For a lot of the characters that have not gone epic the answer to a couple, if not all three, of the questions the answer would be "no."

There's a few destroyed characters in the story who could be waiting to 'go epic'

From memory Nightmare was stomped to death in Colossals. Not sure if he's been seen since.

Warjacks and warbeasts can be advance with their masters. It is somewhat weird that they are stay still while their masters are have one or two changes. Also some warjacks among them are cut in half or even almost scrapped in the history as well, so fix and upgrade it would be a thing. Perhaps CID would be an answer for this, though.

And, although we can't expect too much changes, but intelligent warbeasts and warjack grade cortex can adapt some new weapons as well. I don't think that it is a wise idea to give Ol'Rowdy or Nightmare a long ranged anti armor gun, but give them a short ranged assault ranged weapon and they will make use of it well, because they will like to charge the enemy anyways.

But... it would be a reason to not release the character warjack/beasts: PP never wants the version 2 or the following would be simply an upgrade of the former, since Warmachine: Apotheosis where Epic Warcasters are initially released. It would be not so difficult to make a diffrence with each versions of warcaster/lock, because they can alt the spellsets and feats, and sometimes tweak the weapons. But how they can make a diffrence but not makes the version 2 warjack/beast are simply an upgraded version? It would be too difficult.