Why is Cinderella (usually) the 'leader' or standing in the center of the DP:s? Snow White (1937) is the most classic one, right?

Because Cinderella is a top merchandise seller right next to Ariel and rapunzel...the kids love her more than snow

I think it's because Walt Disney's favorite princess was Cinderella.

I think because she was Walt's favorite and she's popular.

I think it's also because Cinderella is more iconic than Snow White (even though both are). And more marketable.

Maybe it's because she's been the most popular princess for a long time- she's pretty much the icon of Disney alongside Mickey and Tinkerbell.