One reason is that longtime users of Microsoft's ubiquitous desktop software and Windows servers are sticking with the company they've known for decades, now that its Azure cloud technology is ready for prime time.
Companies like Sapient Consulting (a part of advertising giant Publicis), software developer Bentley Systems and biopharmaceutical services company Parexel have all chosen Azure over AWS.
"With Microsoft, their pedigree is as an enterprise software company," said Arun Karur, a vice president at Publicis' Sapient Consulting organization, in an interview with CNBC. About two years ago, executives chose to centralize public cloud use on Azure, and Sapient Consulting has plans to move over all of the "legacy solutions that were on AWS," Karur said.
(Publicis Groupe later clarified that the full company has not officially chosen a single cloud provider.)
To date, Amazon hasn't been hampered by its short history in the enterprise. The company pioneered cloud infrastructure in 2006, letting developers quickly sign up to build websites, apps and run workloads by renting compute and storage capacity rather than buying their own hardware.
Over the past decade, AWS has built a global sales force and bolstered its service agreements to meet the demands of large companies.