1. Fable is now an established franchise from a legendary designer, with a built in audience. LBP is a new franchise that few people have had a chance to try, thus initial wariness is justified
2. Fable is an RPG and RPG's make up a sizable niche of console gaming. LBP is a 2D platformer, which in today's market is a very small niche.
3. The 360's audience is still substanially larger than the PS3s.
4. Sony's base has been built around the "anti-kiddie" image. LBP's image is anathema to much of that base.