I think if Cardassia was more akin to the federation or even the society we have today, Garak might have been held responsible for his actions in the past. But obviously, Cardassia isn't the federation.
The problem with trying anyone like Garak would be twofold: first, you would need to find someone willing to come forward. Parmak is something of an exception to the rule. Just about any person Garak has met from his 'old days' has been terrified of him. Everything else about him can't really be substantiated. The only time Garak seemed to get into legal trouble before his exile was over Procal Dukat's death, and there likely isn't anyone from the Dukat family willing to dig that up again.
The other problem is that a vast majority of Cardassian leadership is culpable in some form brutality or another. Even the more honorable Cardassians, like Tekeny, had dark origins. If you're going to put Garak on trial, you're also going to have to dredge up a huge amount of people to go with him- and at a time when Cardassia is weak, and in need of these 'leadership' type individuals.
I think the recent novels have done a good job showing Cardassia as a country marked with guilt. Garak might not have stood trial, but I don't think its a fair reading to say that anyone forgot, least of all Garak. He carries a lot of guilt over the fact that all the 'good' cardassians seem to have been murdered or destroyed, and still he remains.
Finally, you also have to remember Garak's competition. Temet managed to dodge a very ugly rape trial and was surrounded by scandal. It wasn't like Garak vs. Normal Guy Candidate. During the election I'm sure there was plenty dug up about Garak, but there's probably an equal level of awful surrounding Temet. Garak can at least claim to be repentant. Temet is implied to be trying to cover things up.
As I said before, it really isn't so much that Garak is the right
man for the job. He's just the only one that can currently hold the job long enough for the new generation of Cardassians [IE; Dygan, Dakal, Rugal, ect] to be able to take over.