I think comfort is a major consideration but also appearance. Not all tiaras are pleasant to wear or suited for a person. She had some difficulty balancing the huge ruby piece although it did suit her. She carried the Girls of GB tiara beautifully. But it can also be the other way around: comfortable but not very pleasing to the eye and it can be both and would explain why some pieces will be worn often or remain in the vault or loaned to others. It has nothing to do with pieces being associated with QEII. The Royal Couple have clearly moved on and are not that sentimental, others might want to take their cue.