The main question is: "official" according to whom? Whether or not something is "official" depends very much on which "office" you are asking!
The packages distributed by the MongoDB developers are the "official" packages according to the MongoDB developers. The packages distributed by the Debian developers are the "official" packages according to the Debian developers.
Neither of the two is more "official" than the other in some global sense.
There are many possible reasons why the distribution packages may differ from the vendor packages:
- The vendor packages do not support all architectures the distribution supports. For example, MongoDB only provides packages for Debian on AMD64. But Debian supports not only AMD64 but also armel, armhf, arm64, x86, mipsel, mips64el, ppc64el, and s390x. So, this means that if you use Debian on RaspberryPi (ARM64), there is no package from MongoDB.
- The vendor packages do not support the latest distribution release. The latest release of Debian is Debian 11, but MongoDB only offers packages for Debian 9 and 10.
- The vendor packages are not supported for the same time the distribution is supported. For example, Debian releases are typically supported by the Debian security team for one year after the next release (which usually works out to roughly 3 years). After that, there is a team of volunteers within the Debian community called "Debian LTS", which take over maintenance for up to 5 years after the original release. After that, there is third-party commercial project called "Debian ELTS" which offers support for up to 7 years after the original release. And after that, you can hire a Debian Consultant for additional support, as long as you want.
This means, for example, that Debian 8 still has ELTS support, but there are no packages from MongoDB for it.
- The distribution developers guarantee that every package they ship works together with every other package they ship over the entire lifetime of the release, they guarantee that every bugfix they ship will be backwards-compatible, etc. Typically, vendors do not make the same guarantees for their own packages. E.g. if you use the MongoDB package and an update breaks some random other package, then you will not get support from Debian (because you are not using their package), and it is possible that the MongoDB developers will simply not care about that random other package enough to provide a bugfix. (I am not saying that the MongoDB developers specifically don't care, I'm saying it is possible that the developers of some vendor might not care.)
- Sometimes, vendor packages simply violate some guidelines for how to package software for a particular distribution, so that distributions need to provide their own packages. E.g. some distributions have strict rules about which kinds of files are to be stored in which directories, which directories need to be read-only, etc.
Now, it turns out that in this particular case, Debian has actually stopped providing their own packages because MongoDB changed to a different license. The latest version of the
mongodb package is in Debian 9. Debian itself no longer provides packages for MongoDB in Debian 10, Debian 11, or the under-development Debian 12. However, the package that is in Debian 9 is available for AMD64, ARM64, x86, and PowerPC 64 bit little-endian, whereas the package from the MongoDB developers is available for both Debian 9 and 10, but only for AMD64.
In general, the main reason why distributors provide their own packages is because the distributors have a different set of concerns and constraints than the software vendors, and thus the packages provided by the software vendors often do not satisfy those concerns and constraints.
Remember, most distribution packages are created by unpaid volunteers, and most distributions would like to provide more packages but lack the manpower to do so. If there was a way that they could just take the packages from the software vendors unmodified and drop them into the distribution, they usually would do that, and instead focus their efforts elsewhere.