You've probably seen some press lately about how pervasive Microsoft is in lots of areas (here, here and here). We've also seen SQL Server show up in more and more enterprises - you can read lots of case studies here. There are lots of reasons that you're seeing SQL Server numbers grow - we're less expensive than other offerings, have a pretty good track record and a great toolset to monitor and manage the system. We've also got good support, a well-established repuation in large enterprises and all the way down to small businesses, but there's one factor that I'm seeing make a difference. It's our community.
I've used lots of software over the years (I'm really old) and I've seen lots of community efforts. Open-Source does a great job of keeping people in contact with each other, as do a few other vendors. But I see a LOT of community for SQL Server - people of all levels of skill, all around the world, in multiple industries helping each other out, freely and willingly. Thomas LaRock (SQL Server MVP) recently blogged about this phenomena as well.
I think community makes a difference - if we (Microsoft as a company) listen. And I think we do. I know the folks I deal with constantly monitor social networking sites, blogs, press reports and other sources for a mention of their product area. When they see someone struggling with a feature, they really take that to heart. No, we don't base every product decision on a sampling of people talking, but we sure do listen to what they are talking about.
If you're not connecting with the SQL Server community through social networks, blogs, user groups, SQL Saturdays, PASS or any number of other sources, you're missing out. There's lots of folks out there willing to help - and happy to do it.