That's because Pixar have actual, tangible talent. All they basically got buying Lucasfilm (besides IPs) was ILM, and ILM hasn't done anything in years that couldn't be done as well by anyone else.
And Star Wars simply isn't what it used to be anymore. We're all grown up and disinterested enough not to blindly throw money at the name, and kids now are too young for it (a movie like EpIII was NOT going to turn little kids into lifelong fans).
Buying Star Wars this late in the game is a risk, and the (relatively) low price reflects that.