I'm allowed my own observations, you'll give me that much. Gemma was doing very well for a while, but then Sasha arrived like a powerhouse, working everywhere - and Gemma seemed to fall away. I do find them to be similar-looking girls, therefore my rationale above, that perhaps people were taken more with Sasha.
Personally, I found I grew to like Gemma, then when Sasha arrived, I didn't like her, but when I saw the variety of her work, that changed, and Gemma was the one I wasn't so keen on. She seemed limited in comparison to the spark of Sasha.
But maybe Gemma took a step back to work on other projects - who knows why she's not on the cover of UK Vogue anymore, and Sasha gets it instead.