The first one is good. the second one can be rewritten as: what is it that makes a brand more successful...

How many brands do you want to highlight as being more successful -- one, or more than one? The sentence is plural and highlights more than one brand, and the second is singular and addresses a single brand. But the determiner should also be singular, e.g. "a", rather than the plural "some".

What is it that makes some brands more successful than others? > Ok . "It" refer to any particular reason. What is what makes some brand more successful than others?> Wrong What makes some brands more successful than others? > You are asking for (all) the reasons What is it that makes you like him so much?> It is his good look ) etc What makes you like him so much ? > You can tell him the reasons as much as you want.