It's just because it sounds more showy to people who aren't bassists, which is the majority of the audience.
They think, 'eh wow, how'd you get that sound of a bass. He must be awesome!'.
If you know about the actually difficulty of putting down a solid groove line you're going to apprechiate it more, and hence think it's better. If you don't know about that difficulty the slap line sounds better, and therefore 'is' better.
If that makes sense.