An exact reason for the delay isn't known but Kitguru speculated it could be due to issues with Intel's 14nm process and/or a general lack of competition in the high-end desktop market.
The code-named “Broadwell-E” central processing units for high-end desktops will resemble the existing Core i7-5800-/5900-series “Haswell-E” chips and will even be drop-in compatible with LGA2011 v3 mainboards based on the Intel X99 core-logic. The new HEDT CPUs will feature six or eight cores, up to 20MB of last-level-cache, quad-channel DDR4 memory, PCI Express 3.0 lanes, 140W thermal design power and so on.