That's my guess too.

I only got reality - Spyware and selling Lists makes money.
Money that they cannot make from GoG, so Steam pays more for the same game than GoG,
but they turn around and charge the consumers more for the game.

01. Gamers already bought the game elsewhere, so no real incentive to re-buy.
02. Gamers won't buy GOG version b/c lacks features in other versions - i.e. GOG version might lack MP support (EYE: Divine Cybermancy), gamers on GOG cannot play w/ Steamworks gamers (Dying Light), GOG version lacks Galaxy support (for MP, Cloud Saves, etc), etc.
03. Lacks of sales on GOG versions of games - whether its their own, others, and/or whatever.
04. Developer doesn't want to deal w/ GOG's curated updating process, as it's usually behind other services (where other services allow dev and pub to update how they want and when they want), which can cause gamers to be upset when their GOG version isn't as up-to-date.
05. Developers and pub's will likely release the game LATER to GOG, after they are DONE updating, fixing, revamping other versions on other services.
06. Game is already on their own 1st part subscription-service, so...they want to make sure the catalogue looks good so people sub to that service.
07. Dev's and pub's want extra content left server-side or regulated by client-app junk, so the game doesn't come here or gets kicked off here (Hitman: GOTY - 2016 Game).
08. Dev's and pub's want to utilize always-online DRM, Denuvo, and/or other DRM jun & fear piracy.
09. Licenses is up for music, cars, and/or any other content in-game...so the game can't be re-sold unless the game's re-worked without, all licensed content's pulled, and/or any other non-sense - i.e. think racings games like Need For Speed and/or Forza.
10. IP owners and rights owners cannot be legally squared away - see NOLF 1, 2, and Contract JACK.
11. Some other company buys the IP, so new IP owner would rather put the game elsewhere.
12. Remaster and/or Remake incoming, so the old-version will be pulled from stores and/or only bundled with new version going forward.

You missed apathy.

Or maybe you couldn't be bothered to include more than 10 reasons when there are obviously 18. :)

If like Duke Nukem after the franchise was sold it was pulled off GoG, its simply the new owner does not like the concept of non-DRM games.

Oh they will make excuses to avoid saying that just like they made excuses to switch to console games only which later ends up on the PC platform anyway. Just because a business says something does not mean its true.

Gotta point out Red, There are many games both on GoG and Epic and other PC client DRM.

Well, if the developer doesn't give a damn about updating their game here, it's unlikely that they were satisfied with its sales, so I would put it under the nebulous "lack of sales".

The point is Epic does not make such decisions, the game publisher does or are you going blame Gearbox's anti-no drm stance on Epic as well?

Epic and any other DRM launcher Corpo. ask for higher prices for games AND sell the spyware data, WIN WIN.

Also Alien:Isolation was free twice on Epic and was never taken down on GoG so that blows the theory out of the water.

Junk abandon-ware that is too old to sell is low hanging fruit. "Come use my DRM Launcher".

Wolfenstein: The New Order was also was free on Epic and has not been taken down on GoG

I am completely lost. I don't see how that has any relevance to what I wrote, sorry.
It appears you are reading what I wrote and then assuming things beyond what I actually said, and not actually reading properly what I did write. In other words, jumping to wrong conclusions.
Remember, what isn't said is often as important as what is said.

Quite a few of the Epic giveaways did not require the launcher to play but I am sure you have more anti-logic to explain that away.
Lol, Yes I agree. You are completely lost. But good job defeating yourself here.