Long story: Intel dumped Pentium 4 for Core 2, which had much better instructions per clock. Then going to Nehalem they introduced internal memory controller (which earlier gave AMD the upper hand with Athlon 64 X2 ). Sandy Bridge made further imrovements to the architecture. After that Intel didn't boost performance much, but reduced power consumption with the die shrinks.
Meanwhile AMD had K10, did a K10.5 with tiny improvement, then dumped it for Bulldozer, which has 2 integer cores, but they share other components of the CPU. It was a disappointment. The following generations haven't improved enough.
Intel is way ahead on single-threaded performance, and many applications still do not use parallel code. To get similar from AMD, you needed the FX-9000 series with insanely high power consumption.