Mainly because like all heavy elements (anything past bismuth) their nuclei are so large that they are not stable and fall apart into smaller, more stable elements. Most notably a small helium nucleus is likely to form within the larger nuclear and be ejected. Most astatine decays too fast for it to be useful for anything and francium is so violently reactive with water that even before it has a chance to decay it will probably react with the moisture in the air
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