There are a number of reasons for this. For starters, Cadabra's
notebook interface predates IPython (that is, a publically
available version of it) by quite a few years. That by itself is
of course not a very good reason to keep developing the Cadabra
notebook interface, but it does play a certain role.
A more important reason has to do with the fact that there are,
despite what one might think at first inspection, quite a few
differences between IPython/Jupyter and the Cadabra interface. The
most obvious one is that Cadabra is a native application, which
does not rely on a web browser to display its results. I still
think that native applications have their place, as they tend to
look and feel much better than web apps.
IPython/Jupyter tend to be focussed a lot on data-centric
problems, and less so on being a nice front-end for symbolic
mathematics. These two do differ.
Another reason to keep developing the Cadabra notebook
interface is that it allows me much more flexibility in trying new
features and ideas. As a consequence, Cadabra for instance has a
way to open multiple views on the same notebook.