Because not everyone was quick enough, to get their hands on great titles back then.
Rule: it must be any game before PS3 and X360 console generation.
My personal biggest loss was this:
[spoiler]https://i.kinja-img.com/gawker-media/image/upload/ulcnfctdswrwb2cosuz1.jpg
The Legacy Of Kain series.
I love 3rd person action adventure, yet I never got my hands on it. Only played a bit of Soul Reaver 1, but never beat it.
Maybe it’s time to make that up with Steam summer sale
[/spoiler]