As Zapier has grown, they've had to evolve their software stack for reasons that you likely wouldn't have thought 🤔
Zapier's team absolutely loved Help Scout, although they simply grew to a size and scale to where Help Scout couldn't keep up with the API requests that Zapier required.
For that reason, they switched over to Zendesk, many on the support team are still sad about it, but with the size and growth scale that Zapier had, they didn't have any choice (easily many millions of support tickets per month).