Some companies switch from SAS or IBM because of the cost.
Some people choose Microsoft data mining tools (Data mining add-ins) because they provide enough functionality and are easy to learn.
Very often companies choose data mining tool that integrates with the rest of their BI stack. So, Microsoft shop might choose SQL server data mining because they can use integrated authentication, don't need to replicate the data etc.
I don't think data scientists (people who use data mining tools) switch very often from one tool to another. Usually you find your favorite tool and stick with it.
If you currently use Microsoft products, besides SQL Server you can look at Predixion (which integrates with Microsoft BI tools, Microsoft Excel, allows you to use R machine learning library, has very good visualizations and collaboration capabilities).
Microsoft is conducting an online survey to understand your opinion of the Technet Web site. If you choose to participate, the online survey will be presented to you when you leave the Technet Web site.