Mine tend to be fairly memorable just because I reroll the same families so much. Honestly, it has gotten worse with how much easier it is to save Sims and their families, but even in the good old days I would reroll families for a new adventure. I spend all that time making and tweaking, so I want to reuse them. I like the general characteristics they have and would probably (and have) rehashed them if I was forced to remake them.
I do still switch it up, but the more I play with the same Sims the more endeared they are to me and the more I remember them. I could even say that I have made different versions of a main character from 2-4. Maybe even 1, too, at least once.
The ones that tend to fade from my memory are inspirations that have had their stories played out. Even long, grand stories eventually come to an end and begin to fade from your mind. I was confronted by that when I rescued some of Sims 3 saves from a ghost of PCs past. I remembered the details when checking them out, but I had a fair bit of closure with those old saves.