What is the motivation behind requiring LPGA Tour members to speak English?

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."

via ESPN.

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."

via Golf.com.

Of course, the rule has since being revoked, which was mentioned in the comments.