If I were to try to come at this honestly, my view of it would probably reflect my view on a lot of things in terms of today compared to yester year....
One, you're seeing more about it on DP because there's more users and more posters then there was anytime during Bush's presidency. More people tends to mean more people buying into stupid ginned up "controversies" meaning more posts about it.
Two, at least from my recollection, Michelle seems to be far more active in the public eye than Laura was by and large, or at the very least gets a lot more media attention surrounding here. For instance, the entire thing regarding the public face of the "My Plate" rework of the Food Pyramid and "Let's Move" campaign, and the public promotion campaign that kind of went along with it, is unlike anything I can remember in terms of scope and media exposure for Laura.
Third, Michelle was getting media attention for some time even before the Presidency. Articles about her in Essense in 2006, multiple appearances in Vanity Fair and People for "Best Dressed" prior to becoming FLOTUS, etc. Her magazine exposure didn't stop after becoming FLOTUS either. Only other First Lady than Hilldawg to land on the cover of Vogue. First person to be on the cover of Better Homes and Gardens for almost half a century. The entire story surrounding her, along with her looks and age, and her own personality, make her far easier to latch onto as a socialite and as a "celebrity" of sorts than someone like Laura Bush. Michelle Obama comes off as a celebrity (and I'm not meaning that as a negative), Laura Bush came off as a Country Club Debutante. One is significant more appealing to cover and promote in the media.
Finally, I think it's just the ongoing trend of Media exposure and Social Communication. Laura is the only First Lady we can even compare in a relatively similar time period...and even then, if you think back to the first half of her Husbands presidency to what we have now it's still starkly different. 24/7 networks were common, but not nearly as prevalent. But more than that social media like Facebook, YouTube, Twitter, and the gigantic blogosphere was a shadow of what's there today. The more space there is for noise, the more noises is generated, and I fear that in the future if we have another charismatic, young, outgoing FLOTUs that we're going to look back wondering if Michelle Obama had it as bad as her.
In this particular case, I don't think there's some inherent greater interest to make her a "favorite target" but rather a symptom of the realities of the world we live and the differences in terms of the personalities and personas of the past first ladies and herself.