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…
Zoom probably had quite a bit of leverage in that deal, certainly more than they’d have with MS or Amazon. Google and MS also have competing products.
I don’t think we need to jump to foul play here. (Besides which, what are we, faithful guardians of MSFT and AMZN revenue? It’s ok if they don’t get every deal.)