Because pride is destructive - for instance, look at what Putin's pride is doing in Ukraine
It is therefore not a positive trait - which means that one cannot be a good person whilst exhibiting it
And I have decided that I most certainly want to be a good person, I am working on this very hard
At the very least pride is anti-social and obnoxious, at most it can be the cause of great evil and suffering
Basically: It is unattractive and does harm and I want to be likeable and benign: that's the type of human I would like to become. Hence I want to be humble rather than full of pride.
And yes, I know that me saying that pride is bad and not for me is prideful, that by saying how bad it is and how much I don't wan't to be it is prideful...
You can't win