Why did Mickey Mantle hit so few doubles?

I've known this for some time and no one has provided an answer.

2B%: [2B]/[H]

Doubles as a percent of hits.  For batters with at least 4,000 AB:

David Ortiz is #1: 26%.

Mickey Mantle #932 out of 1,101: 14.24%.

Maybe when I analyse Mickey's PA that is something I can figure out: why a batter with great power and great speed had so few doubles.

Among 81 batters with 350 home runs through 2010 (Ortiz has 378 through 2011 but I don't have that data yet for all players):

Carlos Delgado is #1: 23.7%.

The Mick is #78.  Only Killbrew, Howard and Cash are lower, none of them with any speed.  For 15 of these 81 sluggers (18.5%) at least 20% of their hits were doubles.  

The obvious answer is that the Yankees instructed Mantle to not try for doubles and triples unless he could make it standing.  This would protect his increasingly fragile legs.

Can anyone document this or document some other explanation?