BBUL was presented in October 2001 and was considered as a key component to reach 8GHz by 2005 and 15GHz by 2007.
package instead of building the die on top of it. Advantages of the BBUL technology include small electrical loop inductance and reduced thermo-mechanical strain imposed on interconnects. Prototypes of BBUL packages in 2001 showed just about one third the height of a common processor package, required only 20 to 30 percent the supply voltage of their counterparts. Shorter C4 interconnects were believed to significantly contribute to higher processor speeds.Intel says that they have decided that BBUL isn't needed yet. But it will become somewhere in the future..
One of the reasons could be because Intel isn't really focusing on the GHz numbers of their processors anymore. Consumers have slowly learned that a higher clockspeed doesn't necessarily provides a huge difference in performance - and this causes the need for another marketing strategy.
More info at Tom's Hardware