Rani is still releasing films which are successful, so it's not like Rani has disappeared while Kareena is ruling the roost. It's just that Kareena has managed to maintain her superstar status, which Rani could not. So the question should be 'How has Kareena managed to maintain her superstar status while Rani could not'. 2 reasons:
1. Kareena reached peak late, very very late. She had some hits and acclaimed performances before 2007, but no one took her seriously as an actress and she certainly wasn't a fan favorite before JWM. Even in 2007, Katrina was clearly the bigger star, and there was more mania surrounding Deepika and Sonam than Kareena. Then she had another dull year in 2008, before finding success with 3I and Kurbaan in 2009. That was the start of her successful era. Rani, on the other hand, not only reached her peak way before 2009, but she also found success and therefore fizzled out by that time. She sorta found her first solo hit with Saathiya in 2002, was a top actress from 2003-2006, before finally fizzling out in 2007.
Generally what happens is that when audiences see you for a very long time, they get bored of you and move on to the next hot thing in town. Since Rani was so overexposed in 2003-2006 (literally starring in every big film of that year with the biggest stars like SRK or biggest banners like YRF/KJo), people got thoroughly sick at the sight of her and moved on to newer actresses. Kareena, on the other hand, had a very dull phase from 2002-2007. Even in 2007, she wasn't the most popular actress (katrina was). Then another dull year in 2008, before finally hitting her stride in 2009. Kareena almost felt like a new face -- that was the time she started to get paired with the Khans or big banners, something Rani has achieved 5 years earlier.
So yes, reaching her peak very late did help her cause.
2. Kareena has maintained herself well. She can easily pass of as a 25 years old even at the age of 40. Rani, on the other hand, started to look old and jaded by as early as 2010 (when she was in her 30s). Let's be honest -- looking hot and glamorous is a huge part of Bollywood. Kareena can play these bubbly ott commercial roles or shake her assets in some glamorous dance -- something Rani can't do convincingly today. Hence Rani is limited to her artsy Mardaani type roles, while Kareena can play commercial roles and therefore still retain her superstar status...
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