Are they so different that it's justified to have so many different distributions? So far I guess that different package manager are the reason that divides the linux community. One may be on KDE and one on GNOME but they can use each other's packages but usually you are bound to one manager
“Are they….justified”?
Somebody thought the need for a new package manager was great enough to spend time creating one. That person at least must think it is justified.
We, the users, have not chosen just one of the options to be the standard. Does that “justify” that they all exist?
In the short term, the popularity of Linux is certainly hurt by the complexity of the ecosystem and the lack of standardization. As a product, it would see better adoption of it were more standardized. Without writing a book about why, there is no doubt about this. The short version is that, today, Linux is many products, none of which can compete as effectively as one would and all of them are impaired by the confusion this causes.
In the longer run though, it is almost certainly one of the great strengths of Linux. Linux is many products and as a result, it can target and effectively fill almost every niche. That is going to make it very hard for alternatives to compete at some point. Once Linux knowledge and Linux applications ( yes, I know ) become more mainstream, this compatibility between options becomes a strength. I can have my own operating system that is just the way I want it, but it still runs Docker and Stream ( as examples ).
Think of the cereal aisle at the grocery store. If I want to introduce a new cereal ( or pasta sauce or whatever ), coming up with one that has 10 flavours is not going to work ( without immense marketing muscle ). None of them will sell well enough and probably all of them will get pulled from store shelves. I would be better off launching one. However, once I have a mature market position, I can have not just the regular version but the whole wheat version, the honey nut version, the cinnamon version, the holiday version , etc. They will collectively make each other stronger and all potentially sell well ( again, think pasta sauce flavours if that makes more sense to you ).
This is why there was The Tesla Roadster at first and now there are the Model S, Model 3, Model X, Model Y, and maybe the Cyber Truck.
Linux is not a “product” though. It is an Open Source program. While any given Linux distributor ( distribution ) may think like I outline above, collectively the Linux market is fragmented. Linux is a mix of commercial, community, and individual interests all scratching their own itch.
I am super interested in Chimera Linux right now and fairly negative towards Ubuntu. This makes me part of your problem though. Chimera Linux makes “Linux” less predictable, more confusing, and more frustrating for new and potential users. Pushing everybody to Ubuntu would be a better market strategy. That said, I personally want to use Chimera Linux and, while I say that I want Linux to succeed, I also secretly hope that Ubuntu will fail. Chimera Linux uses a package manager used by only one other Linux ( and in fact they use different, incompatible versions of it so really they are unique ). Clearly, my priorities are mid-aligned with the premise of your question.
So, what does “justified” mean in the Linux space.