When most of us started drinking bourbon, we succumbed to the MM image and thought it was the best bourbon out there. And compared to JB and JD, it was. But, after we started trying the wider variety of bourbons (primarily, after joining SB.com!), we may have felt silly having spent all that money and heaping all that praise on MM.
Also, I think that many of us still love the MM marketing and we get frustrated that MM won't bottle a product worthy of its reputation.
In the two ways above, we feel jilted. Consequently, we sometimes despise our "ex" and tend to bash it.
Anyhow, that's my theory and I am sticking to it.