Why? Is it because of cost? Perhaps nepotism? I don' see a reason why Amazon or Azure would be passed over in favour of Oracle. Why wasn't GCP a contender either? Something seems fishy... someone from Zoom care to chime in? Maybe they are afraid that Amazon or Microsoft with their tradition of copying competition would pose a threat? Even then, Microsoft is already competing using Microsoft Teams and if Amazon wanted…
For Amazon marketplace merchants, I get how Amazon using internal metrics to start carrying popular products is a problem. For Zoom, though? They're public, and they announce quarterly usage numbers. There's nothing to copy that they can't already see.