Microsoft can't get any love these days...
Microsoft is drowning in the mobile market, fighting toe to toe in cloud services, and watching their core business evaporate. Any Linux contribution at this point needs to help drive customers their way. Supporting Linux on Azure is a nice step to diversify their portfolio to reach additional customers, but developing any other Linux software may only serve to help the likes of Google's Android or Apple. They can't have that. Not now.
Notice some of the biggest contributors are hardware manufactures or those with huge hardware partners. Microsoft is still predominately a software company. When most of the open source software is being published under GPL and other copyleft licences, it doesn't bother the hardware folk because they aren't making the money back on the software. If you're a software-centric firm it can really make a huge difference.
But hasn't Microsoft done enough? Oracle buys Sun and then sues Google for using Java in Android... It's amazing Microsoft never cried foul when the mono-project (and others) cropped up to map much of their .NET technologies for the Penguins to enjoy. No lawsuits. If anything, they handed hordes of code over and made several public specifications to encourage it's development. Sure it helped them some indirectly, but would Apple have done the same? Would Google? Ahhhh... No! Think of how useful the mono-project has been to the Linux world. Microsoft not suing earns them a Penguin badge, never mind their past transgressions.