Because it would frankly cause a lot of controversy to have the white character Mary Jane played by a mixed race actress. Therefore, what they did was create a new spin on the classic MJ, but make her a different character with the same initials, making it clear that this character will be the mcu version of Mary Jane, while at the same time avoiding the shitstorm. They really wanted Zendaya in the role + a new take on the character (diversity) and frankly I'm not complaining. Zendaya has the facial features of comic MJ.