If you're a company providing services to people that already have data stored in VendorA's cloud, being on a different cloud would be expensive and prevent you from winning much work. If it turns out that VendorA happens to be the vendor for your clients, you build your services to run on VendorA's cloud too.
This is the situation for my company that started with the intent of being platform agnostic, but it quickly became much less complex as all of the potential client pool was using the same cloud. People with buckets with large amounts of data are not going to be able to convince the bean counters that it would be worth it to have that storage bill from multiple vendors.
> are not going to be able to convince the bean counters that it would be worth it to have that storage bill from multiple vendors
Because it rarely is. Occasional downtime is just a cost of doing business. It is, or should be, rare enough that you just take it as it comes instead of trying to have a redundancy. We don't build tunnels everywhere as a backup for surface roads on snowy days. We just cancel school and work for the day and make up for it later. Do some important things get impacted? Sure, but most things are as mission critical as we make them out to be. The press coverage of an AWS outage makes it so easy to shrug it off and point fingers.
This is the situation for my company that started with the intent of being platform agnostic, but it quickly became much less complex as all of the potential client pool was using the same cloud. People with buckets with large amounts of data are not going to be able to convince the bean counters that it would be worth it to have that storage bill from multiple vendors.