One thing that Matrix does do well is scale. The matrix.org homeserver handles around several million monthly active users, and rooms can scale up to hundreds of thousands of users (although things currently get a bit sluggish if there are more than a few thousand servers participating in a single room).

Meanwhile, usability has improved radically on Matrix clients over the last few years (although many people don't realise if they're stuck on the old legacy Element clients, rather than using newer clients like Element X).

So, not sure what the full story is here.

If they did use Matrix, someone would have to run the server - and Element doesn't have the $ to provide hosting for free, and perhaps k8s doesn't want the admin hassle of running their own. One solution could be for Element to provide a cheap license for the Element Server Suite enterprise distro (https://element.io/server-suite), which would then (ironically) use k8s to scale elastically and be supported by Element's SRE team. It would also have the pleasant side-effect of helping fund Element's development on Matrix and improve things further.

If this is of interest to anyone reading this, you know where to find me :)