RAR isn't an open format and it requires using a different tool with a clunky UI. It's not the end of the world but it's a hassle for very limited file size benefit – Microsoft appears to have gotten in the habit of this back in the day where you needed to create self-extracting archives for people running a version of Windows without integrated ZIP support but now it's a complete anachronism.

The rar files we have at modern.ie are selfextracting. Just follow the instructions and you shouldn't need to install any other tool :)

I've always wondered if there's a story behind modern.ie using chunked .rars over e.g. non-chunked .zip.

What were the technical/human constraints that lead to that solution? Was the team agonising over conflicting goals? Or was it as simple as "Alice already knew it"/"Our tool chain already supported it"?

That's often but not always true: if you use VMWare on Linux or use an OS which is configured not to run arbitrary executables you can't use the self-extracting file. I certainly wouldn't say those are common problems or insurmountable but they're in the class of things which you simply don't need to worry about with ZIP.

What's wrong with .tar.gz like most other *nix software uses for downloads?

I thought 7z supports RAR now?

Setting up that thing is a pain... Definitely could use some work regarding file associations (maybe I'm just doing it wrong).

Putting it into a ZIP allows native support on pretty much every platform.