Compatibility with their zanpaktou helps, too. So does motivation, liek goin' supa sayajin or whatever like a typical shounen anime.>>12088505
Her older brother, remember? He would shut her down if she ever attempted bankai training since he doesn't want her to become a seated officer and be put in real danger. Sending her to Taco Mundo was so that a) he'd get to go himself and b) because he trusts Renji and Ichigo are scared of him enough to fall on a sword for her. *SPOILER* If ever she had the chance though, as shown by her mastery of a new release while fighting the 9th Espada, she'd attain and master bankai at a rate that would put all other shinigami to shame.
Rangiku would get a hot catsuit as a bankai, I'm sure of it. She'd suddenly attain it if her shota captain was ever owned in front of her.
Kenpachi can't hear his own zanpaktou due to the roar of his own reiatsu drowning it out, that and the fact he says he doesn't need or want it because relying on someone else's power makes you weak. He'd REALLY have to be in a bad spot and Yachiru would probably have to be in real danger for him to resort to talking to his sword...>>12088728
Agreed, they 'woke the sleeping dragon' so to say. He gets stronger depending on the opponent, and his desire to get even better only makes him more determined when he finds someone more powerful than him.