Because I always end up buying some of what has gone down the most the year before - that's the spirit of rebalancing. Maintaining a fixed AA forces you to buy low and sell high which inevitably means some money goes to things that look "shaky" in the short term, and money comes out of some assets that look "safe".
I don't really care if Europe is shaky for the next year or so. My portfolio is designed to last for decades, so in the long run it should pay off.
And my international funds don't only invest in Europe, they invest all over the world. Europe is just a part.
Finally, by the time Europe looks "safe" for investors, the fund shares will already have risen to reflect that. At that point, my fixed AA will probably force me to trim a little from that asset class.
I never expect stability in my individual asset classes. That's why I maintain a well diversified AA, because while individual asset classes may be quite volatile, the sum total tends to be much less volatile.
Audrey