Hiya LucidCrux,
As someone who spends a decent amount of time in SYW, here's my 2c (complete with terrible analogy):
Sometimes, if a SYW sub-forum moves quickly (QLH and SF/F for example), a thread may get pushed down and down, and onto the next page, and out of sight, even if the OP is still happy for / wanting crits.
I think of SYW like a restaurant -- if I'm walking past a bunch of restaurants and one is buzzing with lots of people inside, I'll probably choose to eat there rather than the restaurant down the street that's empty, even if the empty place has great food and is desperate for business.
That is, people tend to crit threads others are critting, which tend to be the more active, recent ones.
My advice? If you see something older you'd like to crit, send the OP a rep message or PM to ask if they're still looking for crits.