So first of all, the demographics may be at play here.

Second of all, if yahoo is "powered by bing" that basically means that bing in itself did not win anything, they just took an entire existing user base and replaced one engine with another. Its like if secretly all ford engines were replaced with toyota ones, and toyota claims to have gained market share. Its true technically, but on a user level its not users flocking to them, its just the fact of life that happened due to a takeover. Note yahoo allowed paying to get better rankings, so those who used it were already used to shit results.