Annual technology gap of high end road bikes
While not disagreeing with the first answer above I think there are a few more complexities that haven't been addressed.

Changes in bike technology are not linear but rather generational. Component improvements don't happen each year but rather every 3-4 years. Aside from pro and sponsored riders most riders would not see any value in replacing a bike for the next years model and the industry recognises this. Better to sell a new frame together with other new tech as it's released so people actually are getting something new. 

Di2 was first used in the Tour in 2009 and it became commercially available in the same year. In 2011 it was released on Ultegra, which marked the first trickle down. In the last 3 years it has still not been released on 105. Until they are released in 105 it won't be available on mid range bikes and they will sit in the $2,000-$3,000 price range. That change should be due in the next year or two, unless for other commercial reasons Shimano sweat the Ultegra Di2 line and the new 11 speed 105 line for a few more years. 1 year after releasing 11 speed 6800 they released the 6870 Di2 version for Ultegra. They released 105 5800 in 2014.

It's worth noting that in electronic shifting they have no real competition from SRAM or Campi's EPS (as opposed to  mtb where after Shimano released shadow+ SRAM quickly followed and released type 2 and then 1x11) so will continue to milk their mid range Ultegra until they see sales of that dwindling.

Electric gears will never be standard because there is a cost/ weight/ don't care ratio to consider. There's a point where most people won't be willing to pay (or maintain) an electric shifting system.