You could just look this up. They renamed to Basecamp because they decided to be a single-product company (at the same time, they divested Highrise and Campfire). Six years later, they launched HEY, their email product, so "Basecamp" stopped making sense as a name. They wrote a post about this last year.

later

I added "six years later", but I don't think it changes the meaning of what I wrote originally.

People were used to calling them 37signals, so even if that's the sole reason they renamed, it's more complex than that

Most HN people probably never stopped calling them 37signals, so this seems like an especially weird thing to get hung up on.

Are they dead-naming a company? Isn't that illegal in California already?

I don’t think they were ever Basecamp Inc, I think they were always an LLC.

That decision was always baffling to me. Basecamp is such a UX nightmare even Jira looks good next to it...