Why MS SQL For Big PACS?

It is probably a varying mix of "sell what you know", perceived reliability, compatibility, and some other factors. If it's a product built for a Microsoft OS, there's a good chance the architects/software engineers are familiar and comfortable with Microsoft SQL Server. I'm guessing *nix products typically utilize Postgres, MySQL, or Oracle.

Depending on the application requirements, you may not need to worry about cost though since SQL Express is quite good as long as the data stays under 10GB. There are obviously some features not available in the free version but they aren't always needed/used.

What do you consider Mongo?  Curious in what it is perceived as i have heard a platform changing to it.

SQL handles a larger cardholder and transactional database.  

Many Enterprise level companies have SQL and adding an additional “seat” isn’t as expensive. 

Tools available to repair a full SQL database can become crucial.

At least, that was my experience from a few years back. 

The data portion, even on an average enterprise PACS (to me this means 100-200 doors, under 50,000 Cardholders) , is relatively small compared to the max size (10GB) of a database in SQL Express. Even at those quantities, with a really long retention time for data (2 - 5 years), i would recommend building a secondary Manager Service for load balancing, and add in a second SQL Express for 20GB of data retention. We're talking about actual Bytes or single digit kBytes of data in SQL data entries.