I think there is the nostalgia aspect. I remember growing up reading Peanuts comics. I had a Woodstock stuffed animal. It brings back happy memories, something Hello Kitty doesn't do for me since it wasn't a part of my childhood.
This is what I remember as well. I grew up with him and so did my kids and now when the holidays come around we all still watch Snoopy and now my grandson does. Snoopy is a classic. A very happy memory.