My thought are this,
(a), those who are holding have been in the market for so long already that they know that this strategy is the best (b), perhaps there are long term holders who sell, maybe they needed some money for emergency (c), newbie tries to hold as long as they can, unfortunately, they still don't have the mental toughness and maybe after a month or two or even six months they sell.
However, we all know that buying thru DCA and HODLing is the one-two punch in crypto. It's already been proven by some more senior and early adopters that no one can beat this strategy and so we should learn from them.