Git is not insufficient. For various reasons, Git is not a good match for what OpenBSD needs. OpenBSD needs an implementation that uses privsep, pledge, unveil, fits the mindset needed for Theo to accept running it on his own infrastructure, doesn't carry more baggage than necessary, and is a joy and easy to work on for OpenBSD developers independently from third parties. So the options were forking Git or writing something else, and I chose to do the latter.
See the goals page for more: https://gameoftrees.org/goals.html