Manjaro uses the binary packages prepared by Arch but a distro is more than just a set of packages. (In fact a distro should be more than just copying packages, otherwise it wouldn’t be worth being called a distinct distro.)

Arch’s goal is to be an ultra-customizable distro. To this end it starts out extremely minimalistic and requires the user to “assemble” it during the install from basic components, just so it doesn’t end up with anything that’s not wanted.

If a user can do this then they’re above average in experience and knowledge; and since Arch can reliably assume this about its users it doesn’t coddle them. The maintainers can afford to issue breaking changes that may even go as far as render your install non-operational, because they know their users can deal with it.

Another big Arch feature is being a rolling-release distro and bleeding-edge. This means that packages are released as fast as their developers can make them. This means they often have new bugs. This is the price users pay for the privilege of having very fresh software all the time.

Manjaro prioritizes a safe environment for the user and a more stable experience, where the install doesn’t break (at all, if possible), and can be very easily be restored if it should break. And as a consequence it attracts users with less experience and Linux knowledge.

However, in order to achieve this Manjaro does some things very differently from Arch:

• It holds back new packages and releases them late®, when the Manjaro curators deem them usable.
• It offers an alternate package manager with a more user-friendly interface.
• It recommends the use of long term stable kernel (LTS) releases and mandates installing crucial drivers (graphical drivers in particular) through its own custom tools.

These differences mean that if a Manjaro user were to ask for help from an Arch crowd, the Arch users can’t reliably help because they have no idea what’s going on on the Manjaro side. They may use older packages and the issue being described was fixed in a very fresh version. They use tools (the kernel manager, the package manager, the driver manager) that Arch doesn’t have.

Also there’s very little overlap between the average Manjaro and Arch userbase. If an Arch user is more experienced and the Manjaro user isn’t they’re going to have trouble relating to each other. The Arch user doesn’t see an issue in some occasional breakage, whereas a Manjaro user might consider that unacceptable and so on.

Last but not least there’s a purely technical reason – Manjaro not only delays packages but hosts them in their own repositories, and sometimes goes as far as changing them. This makes it literally “not Arch” – using distinct repos is a step too far in terms of distro heritage.