Official statement :
"Why now? Athletes now have more responsibilities and we want to help their
professional development," deputy commissioner Libba Galloway told The
Associated Press. "There are more fans, more media and more sponsors. We
want to help our athletes as best we can succeed off the golf course as well
as on it."
Reference : http://sports.espn.go.com/golf/news/story?id=3553888
Another official statement :
LPGA Deputy Commissioner Libba Galloway, a graduate of Duke Law and a
practicing attorney before joining the LPGA, disagrees: "We are not
discouraging players from speaking other languages. They can talk to their
caddie in whatever language they choose. They can speak to other players on
the driving range in whatever language they choose. If they're Brazilian and
a reporter asks them a question in Portuguese, by all means, answer it in
Portuguese. And we're not demanding that the players be perfectly fluent in
English. What we're saying is that the ability to speak to your pro-am
partners and to the media, and for the winner to give their victory speech
in English, will be one of our tournament regulations."
Reference : http://www.golf.com/tour-and-news/legal-questions-surround-lpgas-new-rule-requiring-players-speak-english
Going beyond the Official statements, we can only guess on the real issues.
Of course, the rule has since being revoked, which was mentioned in the comments.