Reason why is because the game is shutting down, see announcement from the developers:
https://store.steampowered.com/newshub/app/411540/view/2196011223094683702

There's no reason given in that post.

Edit: Announcement of the closure of Hybrid Wars
The early phase of the sunset begins for Hybrid Wars. Our agents have taken early retirement and the Mechs are shutting down. We want to thank you for your support and believe it is important to give you this information as early as possible so you know what is going on.

What does this all mean? When this announcement was made, all items were removed from sale and the game was hidden on Steam. Support for the game will cease on 30 June 2020. But there's good news: you can continue to play Hybrid Wars and all additional content will be available to all players for free!

Machines engage!

That is true, but the reason why the game is being delisted is because it is being shutdown. For why it is shutdown, I guess that only the devs knows.

Hmm, nothing will be missed according to the reviews this game has managed to get. Could have been something worth buying if it was good and didn't require accounts and stuff but it certainly doesn't looks like it.

The game is in my GOG library but I haven't played it yet. It still has a meaningful single-player part, I guess?

The announcement says: "However, there is some good news: you’ll still be able to play Hybrid Wars and all additional content will be available for free to all players!"

I wonder what that additional content means, does it mean that "Ivan" character you could unlock by logging into the publisher's service, and/or something else?

And, will that free additional content arrive also to GOG, not only Steam?

The game will also be delisted from Steam, and right now it has a huge final discount (80% off).

Why didn't GOG users get the same offer?

Because we are GOG. Not the first time we get treated as second class citizens. At least we are getting a heads up about the removal. Kudos to GOG for that, means a lot. Thank you. Cheers

That makes no sense to me. If the development of the game has ended, why delisting it? They can still sell it like thousands of other games that are no longer developed.
Am I missing something?

For that 1 $ it costs on steam I'd even might pick it up here. If the reviews where overwhelmingly good I'd even... ah... I guess this thread would not exist then :P

The usual game collectors that follow games departing digital stores requested a discount and it seems the devs listened. I agree, it would have been nice if this was across all stores and not just Steam.

EDIT: And just as I added it to my cart, it's at 50% off.

Some questions might be answered here:
https://na.wargaming.net/support/en/products/hw/hot-issues/32705

I also asked the publishers via twitter and facebook if the character "Ivan" will still be accessible even in the GOG release. So far you needed to create an account on wargames.net inside the game to be able to access this character (it could also be that you need to start the game in Gaalxy - but I am not sure about that). If they shut down the server it might mean that character will be locked away forever and since so far we did not receive any update on GOG yet this still is worrying me. They promised ALL content would still be available for everybody so that should mean Ivan as well. Will tell you the moment I get reply.

In the above link they only mentioned the character Yoko Takano which always was accessible in the full package release GOG is selling.

I guess that it may be somehow related to the fact that you already have another game(s) published by Wargaming on GOG (Master of Orion series).

That is not exactly true. It is true that ONE additional and optional character is locked away if you don't create an account. The game however is perfectly playable with the main character and the additional characters you get without creating any account.

Btw: I enjoyed the single player campaign and it is not that bad imo. The gameplay is quite shallow but if something is boring or not you need to decide for yourself. It's a fun shooter with simple gameplay and if you like such things you might enjoy it. I'll never base my purchase decisions purely based on reviews and if I had done so I'd missed so many games I really enjoyed. Better wahtch a video with some gameplay footage in it.
I don't own any other game from them except Hybrid Wars and I get the same discount. "Q. Why is the game shutting down?
The number of players and new sales is steadily declining and does not сover operational costs. Therefore, it has been decided to shut down the game."

(From the link I posted above)

I don't have any game from Wargaming in my library, and I don't get any discount when I add Hybrid Wars to my cart.

It seems that this discount is available to owners of any game from Wargaming, including Hybrid Wars.