1) He has no path to victory. The Daily Beast story says the billionaire backers think the strategy is to have him file as an independent (third-party) candidate, win a few really important states and keep Hillary Clinton and Donald Trump (or Ted Cruz) from winning 270 electoral votes. That tosses the race to the House of Representatives, which (in this telling) would surely choose a decorated leader like Mattis over the Democrat and Republican.
In fact, even getting on the ballots of all 50 states is an expensive challenge; some deadlines for filing signatures land next month. And no third-party candidate has won any state for decades — most states are in fact a lock for one of the two major-party candidates.
The other strategy is to somehow get Mattis nominated at the Republican convention. Sorry: Even after the first ballot (should Trump not win right there), Trump and Cruz have almost 1,400 delegates between them already — more than enough to deny anyone else the nomination.
The convention’s just not going to pick a guy who didn’t run, seems imposed on the party’s grass roots by its elite and whose positions on domestic policy are mostly a mystery.