Some people also just don't like JS so there is that.
Another shallow comparison is that LiveView isn't meant to compete with modern progressive SPAs. While it can update the DOM with data from the server and is very good at doing that, it has limitations such as no offline support. Chris (the speaker in the videos) also notes that if your clients require very good latency or a very complex UI (think Google Sheets) then a JS framework will probably better suit your needs. However, if you're building out a majority of websites (CRUD type pages) that a lot of us web developers do, my personal opinion is that LiveView will be able to everything needed.
[1] Announcing LiveView @ ElixirConf 2018 - https://www.youtube.com/watch?v=Z2DU0qLfPIY [2] LiveView Update @ ElixirConf 2019 - https://www.youtube.com/watch?v=XhNv1ikZNLs