I think it just looks damn sexy and feels great. A Russian with a pair of boobies like yours would have me done in no time... And your breasts are the perfect large soft kind that feel sooo good when they wrap around your cock...
There is nothing that tops the feeling of orgasm inside a woman when both parties are at climax, but that's not an option, then to orgasm on breasts is the next best thing, imho.
Personally I ask the same question for a facial. I've never had a desire to orgasm on a woman's face... despite it being the only finish in the majority of porn today...