SQL Server is about environment and ease of use. It has come a long way, and while I wouldn't use it for large scale projects anymore because of cost, it is more then capable of handling the task. SQL Server does have unique features to the Microsoft environment that make it easier to develop with and make it pretty damn robust too. There are too many things to list, but I think many choose it because it is an eco-system, you get SSRS for reporting, SSIS works well for basic to moderate ETL and the basic setup for a simple database a monkey could do. Not to mention the tooling is really pretty damn robust and easy to use (sometimes too easy IMO). Performance wise, it is good when designed for like any database, but if you do stupid things, or do things how you would do them in say Oracle, then you may be asking for trouble. But the same could be said for Postgres or MySQL etc.

Also, don't forget that many companies started with it as part of the Business Server suite and so all their software was built around it for little cost of the database itself. Then they grew into it.

I have spent a long time working in and around it, and respect it overall. Personally, I have moved a lot of clients out of SQL Server into more cost effective solutions when they have grown to a point that the licensing cost will outweigh the need to re-architect their system. That usually happens when they realize they need robust replication or a distributed solution which will take some architecture work anyway, and why pay MSFT huge licensing fees and architect for it when you can move to say Postgres and save the licensing fees, just as an example.