It's like saying "Why have Canvas, when we already have SVG?". Or "Why have NNTP, when we already have mailing lists"? Or "Why have Git when we have Subversion"? Or "Why have Linux when we already have BSD"?
The projects are diametrically opposite in their most fundamental ways - with the sole exception that they can both be used for instant messaging. Just like SVG and Canvas both render graphics, and NNTP & SMTP can both be used for discussion forums, and Git & SVN both manage source code. They have completely different tradeoffs; one is not necessarily better or worse than the other; they are just utterly different approaches and can and should coexist based on folks' preferences and requirements.
* Matrix is a decentralised conversation history replication protocol, not a messaging protocol. Architecturally it has no concept of direct messages or store-and-forward messaging for IM; instead everything is a group conversation with a conversation history getting replicated between servers (and their clients). Conversely, XMPP is a message passing system (although it's true you could layer a DAG-based conversation database on top in a XEP, at the expense of compatibility with the rest of XMPP). It's literally the difference between NNTP (Matrix) and SMTP+IMAP (XMPP).
* Matrix is a single monolithic spec, defining precisely what features exist for a given stable release. New features are proposed as PRs to the spec (MSCs), often with competing options, but only one gets ratified and merged. Conversely, XMPP is a cloud of XEPs, with compatibility XEPs published occasionally.
There are obviously other differences (e.g. Matrix mandating E2EE for private conversations (https://matrix.org/blog/2020/05/06/cross-signing-and-end-to-...), XML v. JSON etc) but the two points above are the fundamental differences which mean that you wouldn't be able to build Matrix on XMPP short of effectively creating a new protocol - at which point you've effectively built Matrix anyway.
This said, there are also some similarities which often get misrepresented or misunderstood:
* Both protocols are extensible. In Matrix, you can send any data you like (just define a namespace and off you go), and extend existing data with anything you like. You can also go wild and define your own APIs, and room versions (i.e. dialects of the protocol) under your own namespace. Matrix provides mechanisms for backwards compatibility & fallback for clients (and servers, in future) which don't speak your dialect. However, it's abundantly clear that when doing so you are going off piste (although you're welcome to propose your extensions as a change to the main spec).
* Both protocols are open standards, maintained by non-profit foundations (https://matrix.org/foundation and https://xmpp.org/about/xmpp-standards-foundation/). Both started off being built by commercial teams (New Vector Ltd and Jabber Inc respectively) before progressively shifting to an open governance model.
Finally, the weirdest thing about this random IETF mailing list post popping up from April 2021 is that I believe the IETF chose to go with Zulip in the end: their tools team was freaked out that Matrix was openly federating and replicating history from non-IETF servers onto their instance (plus Synapse's admin UI is lacking). On the Matrix side, our solution to this will obviously be to work with the Zulip folks to get Zulip talking Matrix :)
(Disclaimer: i work on Matrix).