why does desire of mortal sin vary so much among people with some wanting to commit every sins while others will only commit certain sins?
It’s mental conditioning.
If you look into yourself, you can see where your desire to commit certain sins came from.
Anger, for instance, is usually frustrated desire. It comes from a discrepancy between the way we want the world to be, and the way the world actually is. Often times, we then look outside of ourselves to find someone or something that we can blame for that discrepancy, and we project the anger onto them.
Then the question becomes, where did the desire come from? That can be investigated too.
Maybe someone insults us. For instance, they call us “dumb.” We project a mental image of ourselves as being smart (a sense of self, the object of the desire). We want that mental image to be real (the desire itself). Someone calls us dumb, which is an affront to this sense of self (the discrepancy between desire and reality). We then feel anger (frustrated desire caused by the discrepancy).
So in that example, the root of the anger is the mental image (and its associated desire), and the attribution of worth to other people’s opinions. If either of those two were not present, the anger would not manifest. All of that is mental conditioning.
Other sins and temptations to sin can be understood in similar ways if you look into their causes.