I always wonder how much the split between .deb and .rpm had to do with the names. They serve the exact same function to the point where there is translation software from one to the other and vice versa, but the names themselves promote the distros they were designed for. Is that the biggest reason for the split? Of course the debian project managed the development of .debs and Red Hat managed the development of rpms, so control of that development path was probably another reason. But this is one split in the linux community that really feels unnecessary.
.deb predates RPM's by a few years, so it's probably more then just the name.
While they accomplish the same thing, RPM's are actually pretty different from .deb since they're binary files. There's more metadata encoded in the package themselves, as well as a built in signature. .debs are, at their core, still just fancy tarballs organized in a specific manner.
Honestly though, RPM's are kind of a mess and pretty difficult to implement without using one of the reference programs, along with containing a lot of extraneous info that you need to fill out. There's a great blog post out there describing someone's attempt to create RPM's from scratch and their gradual descent into insanity. Though, the extra metadata has it's features (better search, dependency resolution, etc).
It has probably been 20 years since I did anything with deb and rpm, but I think this still holds true:
The deb and rpm formats had a very different philosophy to handling dependencies: the deb format relies completely on package names, while the rpm format also allows you depend on the existing of files (or other things?). That implies that the doing/deb ecosystem only works if you manage the entire OS, while rpm (at least theoretically) allows you to use rpms in a system managed in some other way. Of course that capability in the package format itself did not mean people used that.
The file formats are also slightly different: dpkg/deb is completely build using tar and ar, while rpm is a custom binary based on cpio. That also made the deb format flexible enough to not have needed any changes in 30 years, which is pretty amazing.
The source formats have a lot more differences, but I don’t know enough about those. From what I remember srpm used to be more powerful, but Debian’s source format has caught up by now.
A nice side effect of only Debian using dpkg/deb, at least historically, was that when you got a deb file it was guaranteed to work on your Debian system, but for an rpm you could never predict if it would work on your RedHat/SuSe/Mandriva/TurboLinux/etc. system.
