Why is the Yankee Stadium also a football/soccer stadium ?

It is because the soccer team don't have their own stadium yet. And in addition, their ability of converting the field to fit two different sports can be considered as using their resource effectively for accomplishing different goals.

Because it can be converted to accommodate additional promotions

And

Yankee Stadium is also home to NYCFC of the MLS. An American football game, the Pinstripe Bowl, has also happened in the stadium.

No

Yes