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.