Well, you answered your own question here--Turkey has been criticized repeatedly for over-zealous operations against Kurds (e.g. https://www.economist.com/middle-east-and-africa/2019/10/15/turks-cheer-erdogans-war-against-syrian-kurds), so now is a perfect time for it to act as it wishes without too much scrutiny.

Your premise is faulty--in fact, very many people do denounce disproportionate aggression every single time it occurs.

And your premise is faulty in a second way--it is not only attacking sovereign territory that is wrong, but it is causing human suffering that is wrong. Rarely, regrettably, countries use the sanctity of their sovereign territory as a platform from which to inflict massive human suffering either on their own populations or on their neighbors. In such a case, a higher moral value than sovereignty takes precedence: that of humanity.

That Russia's aggression is so shockingly massive that most people have not even noticed what Turkey is up to is all the more reason to condemn Russia, not to let it off the hook. While Russia commits its own horrors of aggression, lesser aggressions are enabled.

After all, "but while you were responding to me, you didn't catch that guy who was beating his wife" is not a very compelling reason for the police to let off a mass-murderer.