This is because DEB and RPM are essentially different ways of doing things and could be seen as a sort of disagreement resulting in two separate systems.
Here is an interesting discussion on the matter:
There is an attempt to standardize on package formats in the Linux Standards Base project, however it seems that Debian was excluded from giving input and a subset of RPM was chosen as the package format. From wikipedia:
Choice of RPM package format
The LSB specifies that software packages should either be delivered as
an LSB-compliant installer, or (preferably) be delivered in a
restricted form of the RPM Package Manager format.
This choice of package format precludes the use of the many other,
existing package formats not compatible with RPM. To address this, the
standard does not dictate what package format the system must use for
its own packages, merely that RPM must be supported to allow packages
from third-party distributors to be installed on a conforming system.
Limitations on Debian
Debian has included optional support for the LSB early on, at version
1.1 in "woody" and 2.0 in "sarge", and later 3.1 in "etch" and 3.2 in "lenny". To use foreign LSB-compliant RPM packages, the end-user needs
to use Debian's Alien program to transform them into the native
package format and then install them.
The LSB-specified RPM format has a restricted subset of RPM
features—to block usage of RPM features that would be untranslatable
to .deb with Alien or other package conversion programs, and vice
versa, as each format has capabilities the other lacks. In practice,
not all Linux binary packages are necessarily LSB-compliant, so while
most can be converted between .rpm and .deb, this operation is
restricted to a subset of packages.
By using Alien, Debian is LSB-compatible for all intents and purposes,
but according to the description of their lsb package, the
presence of the package "does not imply that we believe that Debian
fully complies with the Linux Standard Base, and should not be
construed as a statement that Debian is LSB-compliant."
In general, Debian does strive to comply with the LSB, but there may
be other limitations.
From what I have read the feature sets are so different that it would be very difficult to merge the two. There does seem to at least be a desire to standardize, but if I had to take a bet I would say that Debian's packagae management system will probably eventually take over, simply because of the sheer market share of Ubuntu and some packagers either only doing Debian packages or doing Debian packages first.