I want to add that their attention towards developers is also amazing. I developed an integration for the product of the company I work for, and they were just great; the review process was painless and their APIs are very clean. My only complaint is that the docs can be somewhat lacking
Zapier achieved $35M ARR because they sell $35M worth of customers every year and have a product that can back that sales channel up.
1. Their SEO is on-point (search <Insert SaaS 1> integration with <Insert SaaS 2> and you'll undoubtedly get a 1st page SERP from them
2. Their customer service is great (from what I've read) and appears to be very international friendly.
3. They have good freemium hooks that get you to upgrade and clearly have A/B tested this for conversion purposes.
TL;DR - Product alone doesn't get you $35M ARR.