Having given a zillion interviews, I expect that they weren't looking for the One True Answer, but were interested in seeing if you discussed plausible reasons in an informed way, as well as seeing what areas you focused on (e.g., do you discuss compiler issues or architecture issues). Saying "I dunno" is bad, especially after hints like "what about ..." and spouting complete nonsense is also bad.

(I'm just commenting on interviews in general, and this is in no way a criticism of your response.)