I think it's two things
1. Bing/MSN users are older (and thus probably wealthier) - they got their hotmail account way back when and still use it and even search from within it. Or they search using defaults in I.E. What's more they trust the #1 listing, if it's No. 1 it must be good and they feel comfortable buying. G users are younger and less trusting.
2. There is no Instant on Bing - which means they serve the result only after the searcher has finished telling it exactly what they want - so the match is likely to be closer to what the searcher wants than G's guess based on a couple of letters. G's algo might be fine for the correct phrase, but G interrupts the customer from typing it in. Garbage search query in, garbage result out.