A few reasons:
* They weren't hired or asked to do that
* It doesn't bring more value to the business
* It contributes to a toxic working culture
And it's especially bad in a small company, because you're no longer looking at high level statistics. With only 58 people, you're not saying, "we've got too many white men, statistically." You're implicitly saying, "Mike and Jeff shouldn't have been hired. We should have found people of the race / sex I want to see more of." And, implicitly, "We're going to have to really lean on them to not hire another white guy." Now hiring managers are hesitant to do what's best for the business out of fear of being called -ist.
This idea that employees are entitled to organize privately on company time to try and leverage the company to focus on some unrelated special interest at the expense of being as successful as possible as a business is not as widely accepted as some here seem to think.
It express the reality that having good intentions may have nothing whatsoever to do with outcomes and that misalignment can be exacerbated under more pressure.
The issue at Basecamp and every other socially minded startup is obviously that people don't realize they are pushing on rope, and they lament anyone pointing out the fact as some kind of obstacle.
The clear majority of these kinds of 'social blow ups' are happening at companies where leaders and staff tilt way over to the Left/Liberal and Libertarian/Openness side of things, which should give pause for people consider that maybe those 'pushing back' aren't remotely trump-loving alt-righters, in fact probably the opposite.
The very nature of these threads is exhausting and frankly if I had to work at once of these places I would take respite in not having to deal with it. If there were some kind of pervasive evil that needed to be dealt with, then of course we'd have to do that, but I strongly doubt that this is the case.
These well-meaning D&I enthusiasts are pushing on a rope -- and then, when another innocent man gets killed by the cops the next week or whatever other newsworthy thing happens, they push harder. That's not how ropes work.