Jesus came to be our Savior. To be our substitute. To die FOR us. But while God would be the only one pure enough to die for us... Gods don't die. And while mortals CAN die, no mortal man could ever be the sinless sacrifice the law required to pay for our sins. No ordinary man would be pure and holy enough to take our sins and die for us.
Let's say you are in the hospital and you're scheduled for a serious surgery. And in walks the surgeon. She's a 2nd year nursing student. How many of you would want a 2nd year nursing student to perform surgery on you?
Of course you wouldn't. You want a 40-year-old surgeon with 90 years of experience! There's no way an unqualified surgeon to use a scalpel on you.
Now, let's change the scene: you're in for surgery and in walks the surgeon. He's got the experience. But the moment he walks in the door you know... this man is a pig farmer. He smells of pigs, he's got pig manure on his shoes, his hands haven't been washed in a few days. And he's going to go into surgery like that. Are you going to want him to operate on you? (No). But why not? (He's not clean)
What you want is doctor who is qualified to do the surgery, but clean and pure enough to do it safely. In the same way, Jesus was both mortal enough to die (qualified) and divine enough (pure) to pay the price.