It's part of her brand.
It's like celebs that are careful not to wear certain outfits that would make people question their brand.
e.g. Jennifer Lawrence can't start acting all sophisticated and pretentious because her 'image' is that of a down to earth, real girl that just lucked into Hollywood.
Taylor's is of a girl looking for love and she needs to have regular storylines about her love life because it's what helps sell her albums.

Her being with Harry helps establish them more in America.
It's also good for him in future coz now he has his own big story that will help make him a separate individual from the rest of the band.