Crit damage runes suck early game. Armor pen or AD runes not only give you similar damage crits, but stronger non crits.
Even on Tryndamere, running them means your early game is going to be awful, in exchange for a small boost late game. But when you're already critting hard, critting harder isn't going to make a whole world of difference. That pentakill you got? Doubtful that it was because of crit damage runes.
And by late game, I mean the part where you're farmed to the point where you crit reliably. Which would mean an IE and a dancer/shiv on Trynd, which is going to take you a while to get.
And on top of this limitation, crit damage runes are rediculously expensive in terms of IP cost. They're the biggest newbie trap in the game outside of tier 2 runes.