10% of Microsoft's revenue is Windows. Azure is closer to 40%. Their focus has shifted hard from software to services. Azure is making them around $100B and growing. It'd be far more surprising if they were putting their focus back to Windows in a saturated market given all the incentives they have to chase bigger revenue elsewhere.
We chose Azure for our startup because of data location, and it was cheaper than the other two as well. Data center locations may not matter much in the US (because everyone has this covered well), but it does in other places of the world.
Can't reply to @shorel, but of course Azure is running Linux servers. It has nothing to do with being tied to a Windows ecosystem.
Your choice to go with Azure probably has more to do with where you're getting value for cloud services, and where you started your project - in other words, how tied you are to the Azure ecosystem, or the AWS ecosystem as the case may be.
