There's a few things to realize here about the official rules of the sweepstakes.
As stated earlier, the prizes are determined by a computer. A computer in Elgin, Illinois, to be exact. It goes something like this. Today, the castle suite winner will be the first person to exit from row 5 of vehicle 2 at Star Tours on or after 10:04 a.m. Tomorrow, the castle suite winner will be the first person to take a drink from a specific water fountain in Downtown Disney on or after 11:13 a.m. The next day the castle suite winner will be the first person to walk through a specific exit door at Mickey's PhilharMagic on or after 9:22 a.m. And so on .... These specific mathematic determinations are blind to variables such as race, sex, family/couple/single, and so on.
[The only thing non-random about the awarding of the castle suite prize is that it must be done earlier in the day. This allows time for the winner(s) to get transportation back to their hotel room (or to their home, if they are locals) to pack a bag for the night, to participate in the parade, to have a special dinner, etc. It would be too difficult and also pretty unfair to pick the castle suite winner at 10:30 p.m., which would be too late to enjoy most of the perks of the win.]
Furthermore, when the Dream Squad has any of the "bigger" prizes, they themselves don't even know what prize they are giving out. They just know to inform the proper person and then to escort them to an office or something like that backstage, where someone else informs them of the exact prize. Of course, if the prize is something small and tanglible like Dream FASTPASSes or golden Mickey ears, then the Dream Squad CMs know what that prize is.
I hope this clears some things up.
Like it or not, most (but certainly not all) adult visitors to WDW are there with children.
Like it or not, most (but certainly not all) visitors to WDW are white.
Therefore, just by random luck, most winners are going to be white families.
This is a result of what's generically called "The Laws of Probability," rather than some sinister plot by Disney that most of the winners should be white folks with children.