We wanted to architect a system that made it easy to deploy proxy nodes to multiple regions and clouds. We also wanted it to be easy to add functionality specific to our feature set. While we might have been able to achieve our goals by modifying an existing proxy, it made more sense to us to build our own. I have built proxies in previous companies and this was something I was very comfortable doing.

Can you expand on what specific part of envoy prohibited that?

Additionally, as other commenters mentioned, almost every company has rallied around Envoy and is spending considerable time/money making it better. If your solution isn't as performant as envoy, it seems like a poor architecture choice to roll your own, especially given the time/money constraints startups have.