Why? Wow. . . Because of my 11 girls who began laying 1-2 months ago, I've got one broody, then two, then four, now FIVE broody pullets!! And get this - If they're not brooding on anything - They will NOT stop until something peeps and comes out from under them!
Oh and forget about trying to break them of their broodiness. . . .
Even my Ameraucanas and Marans cannot win the broody contest with my Araucanas. . .
I would say bantams! I have bantam hens and not a day went by this spring/summer when at least one of them wasn't broody. Seriously. The bantam EEs were definitely the broodiest, followed by the bantam wyandottes, and even my silver sebright. Only my bantam barred rock and bantam buff brahmas didn't go broody. I don't know what it is about bantams, but they are broody broody broody!
Forgot the "Why" part. Both are extremely calm, docile, friendly (even while setting) and unlikely to trample eggs. Both are extremely motivated setters. Sometimes I think they would try to hatch rocks if you would let them. After the hatch they are both good mothers.