Same reason I don't like elf on the shelf. I want my kids to want to make good choices because I have taught them well, not because they should be fearful that they aren't getting presents.
There is a fun website where Santa makes a video for your child. Super magical. My 6 year old was beyond excited. I put her older brother on the naughty list and we all had a good laugh. Like, it wasn't personal and it was obviously a joke.