That wasn't really how the Github guys were thinking about their choices, but I'll answer the question as asked...
By raising this round privately instead of going public now, they (a) get a financing partner (us) that completely understands what they're about, wants them to realize their vision, will do anything we can to help them get there, and is in the investment for the long run (10+ years), while (b) avoiding all the pressures and issues that come from being public, which are substantial in the modern era.
By raising this round privately instead of going public now, they (a) get a financing partner (us) that completely understands what they're about, wants them to realize their vision, will do anything we can to help them get there, and is in the investment for the long run (10+ years), while (b) avoiding all the pressures and issues that come from being public, which are substantial in the modern era.