I ran a relatively small web consultancy at the time and we were in the middle of trying to specialise in the new, more complex things people were starting to build on the web.
We had the potential to land a contract with what would be by far the biggest client we'd ever had. But they wanted us to test the water with us first by having us build a series of obnoxiously complex manufacturing cost calculators for one of their sales-focused web sites.
It was all complex rules and interdependent inputs/display values. And because it could all be hard coded it was the perfect candidate for a client-side only site (I don't think we even called them SPAs yet)
I remember thinking jQuery UI didn't feel like the right tool and React was super clunky back then so I reached for Mithril and had a great experience.
The declarative approach, automatic updates to the DOM etc that we take for granted now felt like magic back then. Being able to describe the data model/logic in a relatively abstract way and then watch as the UI changed in response was pretty cool.
I’d bet the customer’s CTO/tech lead took one look at the codebase and was instantly discouraged because some novel and unknown frontend framework was used.