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That's because the world has some sort of really good feature balanced by an even more powerful negative feature. Keep creating worlds until you find a stable age but if you want to cheat there's a command to disable instability. Find it by using /help
You have to be very lucky to make a Mystcraft age with no instability, unless you cheat and use a creative book which generates worlds with no stability issues.
You can also disable this feature completely, though as TheSandwichMakr has said this feature is added for balance, by editing the mystcraft_config file. There should be two settings regarding instability, you can set these to false and you shouldn't see this any more.
You do get slim chances of a random world being stable. Not every world I visited was unstable, or it's instability effect was so minor, it wasn't worth worrying about.