Compare that to the scenario at Marvel, where so far, nobody has stood a chance against MJ. And a big part of that, as you said, is about Spider-Man being about the man under the mask and Superman being about the power and inspiration inevitably attached to such a potent symbol. Lois works as a good love interest because she grounds Supes, but MJ works far better because in addition to being an anchor, her personality and effervescence actually help Peter flesh out his own personality and outlook more so than before. Also, no matter how much people mock her profession and ambitions, you cannot deny that they add more colors and diversity to the tone of the series. MJ brings the glamor and wonder to Spider-Man, whereas Lois works in the same field as Clark, so she doesn't bring anything new there.
On a more shallow note, MJ also has the whole "wish fulfillment" thing going for her, which has always had a powerful pull when it comes to fiction featuring underdogs.