Scarcity of electric parts: motors etc
Cost. If you look at any set with electric motors, etc. you will see a significant increase in the price of these sets.  They have marketing research groups and accountants that somewhat dictate the price range of the sets they release.  Any engineer with good experience can tell you of the headaches of dealing with keeping production cost down.  Since the cost of the motorized sets are inflated by including expensive to produce/source parts, they will only allow so many to be produced/released each year.  

Most customers take things at face value.  They see that you "get less (pieces)" for your money in these sets and are less likely to buy them as often. The Lego company's targeted demographic has been stated as 8-12 year old boys from various sources.  I tend to believe this claim based on the amount of sets released with those ages listed on them.  This would also suggest that the majority of their marketing research is dedicated to this specific group, and has shown that electric components aren't a big selling point. I can confirm this to be accurate based on my own children and their friends.  

I know from an engineering standpoint that the more components something has, and how "specialized" they are, the more expensive it is to produce. Because of this many parts are outsourced from companies that specialize it certain components. Electronics are good example of this as many of the internal components are manufactured from different companies. So not only are these individual pieces bought from several different places, but then they have to be assembled.  All this equals "cost of production".  

One last note: I often come across sets for sale (Bricklink, Ebay, etc.) without the electronic components.  I have come to the conclusion that these people are buying the sets just for the motors, etc. and selling the rest to recover some of the cost.  This works out well for sets like trains where adding additional cars is a common desire. This is option you might consider.          

Related: How much does an average LEGO brick cost to produce?  

Another possible aspect that I have no knowledge of is if there are any additional costs to import/export products that fall under "electronics".