So a picure is worth a thousand words... I'll try and explain this in less than 1000 words. My brother and I were talking about the BitKeeper incident last week. He claimed that the revocation of the BK license to OSDL and that Linus' subsequent droppage of BK is prototypical for those who don't play fair with open source in the software industry. The OS community will move on. That seems pretty harsh but might not be too far off the mark. (When not using "free" software you are subject to the whims of the vendor, even if their proprietary software makes your job easier).
But enough waxing Stallman. For me it is interesting to observe the catalysts that drive open source adoption as well as those that drive features. For many open source projects, they seem to grow organically, and sometimes that growth happens in large spurts. I'd like to conjecture why (I have no real proof, so feel free to correct me if I'm wrong, this is only based on casual observation).
In this instance, at the point in time I've labeled as the "debacle point", there was no open source software that provided Linus with the functionality that he needed to serve as the filter for kernel patches. This vacuum caused by BK was the catalyst for rapid development of Git and it's adoption among kernel developers.
A few notes. I believe the adoption is aided by the fact that a respected leader such as Linus was the one who developed Git. The adoption graph only represents usage among the kernel developers (I don't know what will happen in general to the adoption of BK, it seems they ostracized their reference client, but maybe in their eyes any news is good news (er marketing) for the BK folks). Git may not ever have all the features of BK, but it will eventually be "good enough". I make no guarantee about the scale of my graphs (I'm just trying to illustrate a point). I was kind of hoping that some of the other nascent distributed SCM projects would step up to the plate (bazaar-ng), and there would be mass improvements as projects competed to be Linus' pet SCM. (was that under 1000 words???)