When Partyhats were released, people did not know they would become rares, so they did not try to hoard them. When they were released, many of them were sold to general stores or dropped since many people thought they were worthless. By the time h'weens and santas were released, people understood the concept of rares so many people tried to grab them as soon as they were dropped. (Also, there were more players in runescape in 2002 versus 2001, so more people were there to pick up the items).
As for the colors, in a perfect world, they would all have the same rarity. However, it turns out that green h'weens must have been picked up more often than blue and red ones. Also, red h'weens were bought more than the other masks due to people thinking it completed a dragon set better than a dragon med helm (dragon full didn't exist). The people who bought the red masks at the time eventually quit as they were spread out throughout the runescape population more than the other masks, so their prices will be higher.People like keeping historical prices as well, they rarely change in terms of ratio, although they do sometimes.
Pumpkins were the first rare actually

And technically, christmas crackers were before party hats.