I think she had some limitations to be all round and consistent enough. She has good results in classic but her technique was more dependent on good grip then others, which made her results in classical style less consistent.
Also she was obviously a good sprinter enough to win an olympic relay and be one of the most successfull team sprinters ever, but in indivual sprints, or in mass starts as most of the world cup races have been she has not been good enough compared to Björgen etc. Better in individual starts and especially freestyle. Around the 2010 olympic she was unbeatable in 10 km freestyle individual start but not at all the same in other disciplines and even if she improved in classic, others cathed her and she lost her advantage as the best technical freestyle skiier as I think she was back then.
She has actually a podcast together with Jonna Sundling, Hanna Öberg and Elvira Öberg where they just recently discussed this. Some listeners questioned why the cross country skiiers and in particular some swedish women doesn´t seem to prioritize the world cup compared to the biathles. Also the Öberg sisters questioned that.
Kalla said she really tried in the beginning of her career but her only sprint win was in that Tour de Ski 2008 and she had to accept she would be better and wouldn´t have the chance to win the overall world cup and then the championships became more important and she become a bit of a championship specialist, especially in olympics where she is the most decorated winter olympic athlete from Sweden (9 medals). but not so many WC wins.
Sundling is a bit similar with the difference that they have different qualities and that Sundling was quite old when she broke through. Much because of many seasons destroyed by sickness and injuries, and she has finally found the recipe on how to succeed at championships and try to avoid the long sickness periods and that she has to try to follow the plan what is best for her even if she pointed out that she felt a bit guilty against the sport when she skips tour de ski and also a few other weekends.