Source: https://cds.cern.ch/record/1407874/files/fc-e-5578-Budget201...
Ctrl+F "level domain"
Edit 2: Additional cool source:
https://cds.cern.ch/record/2296678/files/pdf.pdf
"3. Motivation for gTLD .cern
The motivation for CERN to acquire its own generic top level domain was to be able to better support its Web communication strategy. In addition to the traditional CERN domain .cern.ch, that is used for all IT infrastructure and also for internal Web sites, other benefits of owning its own TLD are in the area of conveying:
- Branding
- Recognition
- Innovation
- Trust"
"As such, a domain that limits CERN to one country doesn’t acknowledge the international, and increasingly global, nature of the organization, nor that its science and values transcend geographical and political boundaries."
CERN has all sorts of stuff going on, I don't know the details but its not just like a normal lab in a single country, there's special tax rules and diplomatic treatment for people who work there etc.
CERN has friendlier funders.
Or, they just thought it’s cool :-). Companies spend all kinds of money on things without ROI: fancy decors and furniture, landscaping and fancy buildings.
It's just part of a wider range of branded TLDs, some of which include .walmart and .mcdonalds