Didn't want to edit/update my last post (unlike some people who change their posts more often than the weather in Kansas!)...

The thing to remember about the history of OLAP (which is tied to much of BI) is the role MS had in the early OLAP specifications.

They certainly didn't invent OLAP, but they were one of the pioneers in publishing OLAP specifications (and they are credited for the MDX specification). But their implementations in the precursor to SSAS 2000 and SQL 2000 itself weren't great, and their database engine in the BI stack just wasn't viewed as powerful enough.

SQL 2005, 2008, and now PDW have changed much of the reactions. The reason they've been gaining so much ground (and I think will continue to) is threefold:

1) They are being taken more seriously as a robust platform
2) Simpler and more cost-effective licensing
3) Increased functionality and integration with Office and SharePoint

Believe me, their BI tools still need improving. The current implementation of metadata management is extremely weak (and the big Gartner study pointed that out). But they are a player, and have definitely knocked a few birds off their perches.