Lucifer needs to feel like he's making up for all the lost time with his daughter after failing her as a father... But their relationship is broken beyond repair. Besides, Charlie is already all grown up, she doesn't need his old man anymore, does she? For this reason he adopted you... The ruler of Hell would start anew with a new child and proove himself he's not an awful parent.

You can't deny that, even thought the circunstances of your adoption are a tad depressing, you have been greatly benefited. Desperate to proove that he's a good dad he spoils you rotten... And, in a way, you are now royalty. You went from being an orphan to being the princess of Hell, from having nothing to owning everything, so... Hooray?