Because the Yanks will be able to get Stanton for very little in terms of talent, essentially taking on the contract. And, he will be a relative bargain compared to Harper in terms of cost while providing excitement equal to Harper. If they paired Harper with a Moustakas acquisition, they would line up:
2B Castro
CF/LF Gardner
DH/RF/LF Stanton
3B Moustakas
RF/LF Judge
SS Gregorious
LF Frazier
C Sanchez
1B Bird
Ellsbury, Romine, Torreyes, Hicks (if they can trade either Ellsbury or Gardner), Tyler Wade and Gleyber Torres
They will trade for a couple of starters to go with Tanaka, Gray, Severino, maybe bring Sabathia back as their 5 on a relatively cheap contract.
They will be shedding big contracts after pretty much every year over the next 3 years and it isn't like they don't spend money anyway.
They will want Moustakas at 3B I think because of their need for another LH power bat to fit into their lineup (and if they get Stanton to split Judge and Stanton).
I think they've been extremely smart and a little lucky in their rebuild.