I use Git instead of SVN (Subversion) because it allows us to scale our development team. At any given time, the Zulip open source project has hundreds of open pull requests from tens of contributors, each in various stages of the pipeline. Git's workflow makes it very easy to context switch between different feature branches.
I use Zulip instead of Slack, Mattermost, or RocketChat because of its first class threading. One week after switching to Gmail (in 2004) I realized I was never (willingly) going to use an unthreaded email product again. I had that same experience the first time I saw Zulip.
Zulip is also fully open-source, with a well-maintained (e.g. 90+% test coverage, fully static python), easily extensible code-base. In many companies, your communication platform (chat or email) is the center of the workplace -- no one asks for a chat integration into their calendar, they ask for a calendar integration into their chat. A fully open-source codebase means you can customize Zulip to your needs, and are never at the whim of a corporate maintainer who can't or won't fix simple bugs, or who will charge you tens of thousands of dollars for making minor customizations.
i use Stripe for payments and billing because the API and documentation is well thought through, and the web UI is great for getting a running start. We spent about a month writing a mocking and testing framework for billing with Stripe (all a part the Zulip open source project); join us on https://chat.zulip.org if you're considering integrating with Stripe and need some code or advice!
I use Front instead of Zendesk because even in very early beta it had gotten many of the little details of what we wanted in a customer support product right. E.g. emails look like they come from me, instead of looking like they come from a ticketing system. And they do a good job at basic workflows like assigning tickets, and having side conversations about tickets. Other features I like are their keyboard shortcuts, and canned responses (though I expect other products have those as well).
For context, the Zulip support team is roughly 1-3 people, depending on who you count.
