There are several obstacles to packaging riot-web (both the desktop version and the webapp version) in Debian. In addition to react and electron not being in Debian, as shirish mentioned, the build process needs a newer version of npm than what is available in Debian, and the JavaScript version of olm requires emscripten, which is currently not usable in Debian. There are probably many other little JavaScript packages that Riot depends on that are not yet in Debian.
We have a wiki page tracking the packaging status for several Matrix-related software in Debian, including Riot: https://wiki.debian.org/Matrix Hubert
