I think it's more then just 'political', although it definitely plays a role, particularly in today's supercharged climate wth all the clamoring for more 'diversity'. This is the second reboot in 5 years so it seems like a given that the new filmmakers would want to change the flavor for these films, aside from making Peter younger and having the MCU connection. Making an iconic comics couple biracial would definitely add a new spice (and controversy) to the mix and be exciting to a lot of people, particularly those in the GA who are outside of comicdom. I'm also certain that if Zendaya is indeed MJ that Marvel will have made sure she was right for the part before casting her, much like they did with Tom when they screen tested him with RDJ and Chris Evans.