My short answer is: because chances are that your Filipino wife and, most of all, her extended family (if they live with you) might never be the ones to adjust.
It may appear that, because Filipinos are so well-travelled (my wife for example worked in several Asian countries and in the USA before she came to Italy), they should easily and readily be able to broaden their perspective and embrace other cultures.
What I’ve realized is that, although they have moved their BODIES to a different country, their HEARTS and MINDS STILL LIVE IN THE PHILIPPINES.
They spend their free time chatting with their friends and relatives in the Philippines through Skype or Facebook and associating with the Filipino community in the country where they work and rarely do they widen out and make friends with the people of the host country.
Before I married my wife I travelled abroad less than my wife did, but, like most Westerners, I travelled moved by a spirit of adventure and a burning desire to EXPLORE OTHER CULTURES.
So, I’ve come to the conclusion that, although my wife is the “immigrant” here in Italy, I, as her Western husband, am in a better position to open up to other cultures because I have a background that is made up of years of travelling for the purpose of expanding my mental and cultural horizons while many Filipinos only move their bodies to another country but, emotionally speaking, they never really leave the Philippines.
But, apart from the cultural diversities that exist in a multiethnic marriage, I think that anyone who enters any intimate relationship, interracial or not, should, as a general criterion, always be the first one to make the move of going the extra mile instead of expecting his or her spouse to make the first move because the cardinal rule for a successful marriage of any sort, not necessarily a mixed one, is giving and yielding.