There are a couple of technical issues with Mono, the biggest ones being completeness and tools integration. But, above all, the biggest problem is not technic

There are a couple of technical issues with Mono, the biggest ones being completeness and tools integration. But, above all, the biggest problem is not technical and not Mono itself: it is Xamarin. The money decisions on big companies are made by people that need to tell to non-technical bosses and shareholders why they did what they did. And it is much easier to talk to them about Microsoft than about Xamarin. Remember the "no one ever lost their job for buying IBM"? It works for MS, now.