Same way AMD toys with their GPUs,only releasing firmware to compete with nvidia and not for the sake of making them better.
Its called marketing.
My guess is Intel cannot make Ivy Bridge processors fast enough for them to take over all their sales. This way, with them being marginally better, there still will be a strong demand for the Sandy Bridge ones. If they did make Ivy Bridges able to clock fully, everyone will want Ivy Bridge and Sandy Bridge fabs will be idol.
As of now Intel is really in the driver's seat with their cash on hand, fabs, and technology. But, in a few years we just may be seeing things change with diminishing need for more processors and not needing faster ones for PC's.