Because recreation_ground is defined in the Wiki as "An open space for general recreation, which often includes formal or informal pitches, nets and other recreation equipment or infrastructure" — in other words, a very man-made space, similar to leisure=park.  But Swasey Recreation Area is 99% in a natural state.  "Nature reserve" may not be the perfect tag for the situation, but it's the closest we've got.  (See: https://wiki.openstreetmap.org/wiki/United_States/Public_lands#Bureau_of_Land_Management)