Serious question, for market strategy understanding:

HA would normally use two AZs, 21C architectures would use three AZs, and other patterns such as pilot light let you use additional AZs without a significant cost hit. Further, when you can make workload-handling instances so much smaller (even down into the T sizes, and with cattle patterns you can start to leverage spot pricing), each additional AZ you add to the 21C mix represents a smaller percentage of your capacity lost in an AZ outage.

What factors have you choosing an automation powered CSP such as AWS but using only a single AZ out of a half dozen?

If using a single AZ and tense during outages, why not use Hetzner, Softlayer (now IBM Cloud), etc.?