It is because non natives have no culture, no 'traditions' handed down, except for religion, which, as in the Christian religion, the book has been rewritten a few times, changing this and that, and basically, there isn't 'traditions' handed down.
But Critter, this is simply not true. People who think NDN and Eastern traditions are the only surviving "mystical" traditions are simply not looking in the right places.
Speaking as an Irish-/Scottish-American and Gaelic Polytheist: Those whose families have assimilated into mainstream culture have a lot of work to do. Even those of us who have some fragments surviving in our families, and who didn't have Christianity shoved down our throats, haven't had it easy. We've had to find others who have fragments, and over time put together something more complete. It's taken hard work and patience, but we also have our old people and tradition-bearers. And we have middle-aged people who have been doing this for most of our lives, and young adults who were raised in these fuller, revived traditions.
Many people, of all ages, were raised with the songs and stories and folklore that embody an earth-based, mystical worldview, both in the Six Celtic Nations and the diaspora. For some, maybe most, it was just cultural, but for others it has always been part of their spirituality.
Don't rule out books; there are solid collections of old prayers, songs and folklore, including ancient manuscripts with our mythologies. It's better to learn these things in one of the intact oral traditions, but if people hadn't made the choice to be less secretive and record some of these things, we would have lost even more. These sorts of historical records, and personal stories of our ancestors about their beliefs and experiences and daily lives, are of a whole different order than the occult and newage books that claim to teach spiritual techniques. You probably won't find them on the bookshelf of your local occult/nuage store, and some have never been translated into English, but they exist in University libraries, cultural centers and private collections. Of course you can't learn ceremony and spirit work from a book... though if you already have some training and aptitude, reputable, old books of prayer and song and tales can give you more to contemplate and work with. Sincere prayer and right behaviour can open doors to meeting the right people.
I know people from a variety of European ethnicities who have solid traditions that have survived. They may not all be as full and culturally-integrated and widespread as these sorts of things are in traditional NDN communities, or in a Hindu ashram, but as more and more people have looked to earth-based ways as an alternative to mainstream religions, things have improved. There are also far, far more flakes and frauds than genuine people. But like with anything, what is good and valuable and solid is not going to be found quickly or easily. We don't have to be nuagers or pretendians to have a mystical, earth-based, meaningful spiritual practice.
