They were kind of a default since there's only a few huge ones. Main win: it's not Microsoft. MS already has lots of internal tools from MS Research they were wasting. They probably couldve built Semmle themselves. They're also a known patent troll. I don't know if Semmle's methods are patented, though.

I'd rather a company like them not acquire them in favor of one that is constantly developing new services with no attempts to financially drain 3rd parties. Not to say good won't come out of Github integration given huge number of projects in it.