Because in 10 years, .270WSM brass will be made of 100% Unobtainium, same as the .225 WIN is today.
It's already hard to find on a shelf, and spendier than the .270 WIN when you do find it.
In 10-15 years the 270 and 300 WSM will be the top magnum sellers and move the 300 WM and 7mm Rem mag to 2nd place. The trend is already happening. The other WSM's are history.
But in this case I wouldn't suggest either. The truth is that almost any chambering will work just fine. Nothing at all wrong with 260, 7-08, 243, 308, 270, 280, 25-06, 30-06 or any of about a dozen more chamberings. I'd go gun shopping and select the gun that called my name in any of the above.