…you might come off like a housecat dragging home a kill. You think you’ve given people a treasure, but their only thought might well be, “ew”.
…while you might feel unburdened, they may associate you with unwanted emotional labor, and they have the right to say to themselves, “Nope”
…you might remind them of a version of themselves they dislike.
…in some nondescript way, hearing from you might not feel the same to them as it would have ten, five, or even three years ago.
…they may have just experienced a death or another event that has changed them.
…their worldview may have shifted into something that’s incompatible with (or even harmful to) you.
…they might be of a kind that’s unaccustomed to/falls silent when presented with earnestness, compliments, etc.
…they might have established certain ground rules for themselves, to stay forward-looking and avoid overwhelm, and contacting you might be against those rules.
…could be they actually think of you a lot but prefer you as a hypothetical rather than a known entity.
…you’ve REALLY got to be OK with them not writing back.
If you’ve fully reflected on all that and still want to do it, then do it (but don’t OVERdo it)
(This from someone who has been known to write to people from my past.)