Well, see, I've whittled all the different possibilities for her high ranking, and I've thought of the main two measurements you could base her Waifu-levels off of,they would be her Ability To Engage Parental Instincts (ATEPI, pronounced Aey-Tepi(d)) and her Emotional Tendency in Regards to Protectiveness Towards Allies (ETRPTA, or E-Terp-Tah), both of these are extraordinarily high, her attitude lends itself both to a persons instinctive attitude to protect helpless things, and her loyalty towards who she percieves as her friends allows for a very large crowd for people who value such innocent behaviours, there's also the overwhelming archetype of strength that she fulfils, both of her causes for her high waifu-levels are reinforced by it, the protectiveness she garners as you can see both her strength and weakness, while her loyal attitude is shown ever more birghtly when enforced by someone who could, pardon my language, 'wipe the floor with your sorry ass.'
Now, normally such a lack of perspective towards other people and an incredibly emotionally cut-off personality causes a bit of a lack in interest, but as said before both her background and responses to such a background cause a strong surge of protective attachment, allowing for basic bonding to occur (only strengthened by the natural curiosity that one must feel when confronted with such an enigma) that as such leads into a higher emotional connection with her than other types of warriors as you slowly learn her story, keeping interest up and maximising the time for a true emotional attachment, the 'shy and cute but extraordinarily deadly' archetype is something that has around the same effect, but on a much more blunt level, and without any defining attributes beyond that, interest would tend to wane quickly, remember, staying power is key for any burgeoning waifu, without that, any group you gain would quickly scatter soon after the main reveal of your character, but with Anna, the main reveal of her character is only the beginning.