I don't think Margrethe is not generous to Marie, actually she can't be generous because
Denmark actually doesn't have enough "free" tiaras for Marie or other ladies to choose. Pearl Poire and Emerald Parure are "queen-only", Gold Naasut is a personal gift for Margrethe, Ruby Parure is assigned to Frederick's wife, Diamond Drop is a gift/lifetime loan to Alexandra. Some now belong to the Queen's sisters or sidelines of the family. What remains are [1] Dagmar's Floral, [2] the Turquoise, [3] Floral Aigrette, [4] Baden Palmette, and [5] Gold Poppies. [2] are probably too small and not grand enough, [3] and [4] seem to be Margrethe's favourites and Marie may choose to avoid it? and......you need to be so bold if you want to put [5] on your head. So what Marie can pick is Dagmar's Floral. (btw I think the Diamond Drop might go to Marie if it didn't leave in the family with Alexandra.) OTOH Marie always gives me a feeling that she isn't very interested about jewellery, I don't know it is because she really isn't fond of them or her limited jewellery creates a wrong impression to me.
Actually the situation in DRF is quite similar to BRF, many pieces are reserved for someone or gifted to someone and they don't have the tradition of sharing tiaras. (But of course BRF has enough tiaras in their vault and they don't have many tiara events so so far they don't have a serious problem.)