There are lots of possible reasons why a game gets removed.

Abandonware and no clear legal situation about the owner. No One Lives Forever for example. You cannot buy this game new for that reason.

Legal issues about used assets, licences, other things and legal disputes. Even though some issues could be solved, it leads to the next reason. Only Up has some of these problems.

Stress. Some people do not want to be public and famous. Some viral apps went that way.

The developer has regrets about publishing the game. For various reasons. Flappy Bird was removed for such a reason.

Fake games and impostors. Less likely to happen on Steam, but depending where you find a game, it might not be the real one, and got removed. Some "demo" versions on a free platorm might actually be malware scams.

Lack of time and commitment. If you release a game, you are somewhat responsible for it. Some games here have disabled comment sections because of that. (And others have it disabled, because they are scams). Indie developers are usually more to the ground and approachable, so they might feel uncomfortable to give support for a years old game they maybe not even have source code anymore.

Embarassement or other reasons for not wanting the game to be known. There is this thing with the director's cut in movies. If a creator of a work is unsatisfied, they sometimes try to fix it and sometimes just move on. It is artists and there is this trope of a painter destroying his own paintings.

For releasing on Steam as paid and not on Itch, specifically, there are idiosyncratic reasons. Payments are handled not in a way on itch the same way as on Steam. It is sometimes not possible to sell on itch for a developer, for some reason or another. Remember, that while itch is US based, the developers and users are all around the globe. There are projects here, where you can even get a steam key with your purchase. Of course not releasing on both platforms can have all sorts of reasons. The most trivial can be, that it takes effort to manage several distribution places. And let's be honest, itch is a puddle compared to the steam ocean. A game can have thousands of reviews on steam and have single digit ratings on itch. I kid you not. This game here has 2.8k reviews on steam and 4 ratings on itch.

Of course there are counter exampels. This one has 2k ratings on itch and 5k reviews on steam.

It is difficult to explain, because there are many possible causes, as @redonihunter told you. Furthermore, many are personal.

1. To publish on Steam you need to integrate its API, so you need to have 2 versions of the game, one with the Steam API and another without the API for it to work in Itch. Some people prefer to focus on just one platform and steam is usually a better place to sell.

Some independent developers are afraid of publishing their game without DRM and that is why they prefer not to publish the full game on Itch and only the demo.

Some have never thought about publishing on Itch, but they use it only as a form of promotion and that is why they only post the demo, etc.

2 and 3. This is more complicated, in some cases there are legal reasons, for example, it happens that the publisher is not the creator/owner of the game. It may be that the contract expires, that the publisher closes or is bought by another company, forcing new contracts to be made, which if the original owners cannot or do not want to sign. So the game can no longer be sold.

In the real indie world, many developers sometimes use copyrighted content and are forced to remove the game.

I have seen many developers who prefer to avoid problems and stop offering the game, either because of criticism, or because of some controversy (there are many people who make disputes over very simple things, like a game not having a certain option or if it does have one).

There are people who leave the project aside because it didn't go as well as they expected or because of personal problems and they prefer to delete the game.