If I had to guess why it is, I'd say that back-end technologies naturally evolve slower. Yes, we are in the middle of a Big Data boom, but even then, the rate at which Hadoop changes is nowhere near Ruby on Rails, to cite two popular technologies.
Also, the back-end stuff tends to be a bit less sexy, but also less fault-tolerant. Management sees the front end; the back end is basically magic. So it makes sense that you have Gandalf back there with his beard and recumbent bike banging out magical koans to keep the database alive, while young bucks guard the front gate of the castle.
The back-end in the last five years or so has definitely had a lot of change in data stores, emphasis on RESTful API exposure to front-end and other back-end services, web server choice and style (WSGI, PSGI, FCGI, etc).
The biggest difference is that a front-end developer has to worry about the performance of a single client using a single state machine. The back-end guy has to deal with making a site fast and reliable while supporting 100s or 1000s of those clients concurrently.
Crash the browser every 100K views and someone reloads. Crash the back-end every 100K views and someone gets fired.
In the Dark Past, this last line of defense was in stored procedures, but we -- 30-something greybeards :) -- know what a mess that was. There are plenty of companies who are still tied to an ancient DBMS due to that 10kLOC stored procedure that looked like a good idea at the time.
I think it is this way because back-end development requires more expertise than GUI development. Steady more experienced developers are better suited than new hires.
Slow = bad
Fast = good
They probably think a good GUI is a question of taste and can't be measured and they don't want depend on "luck"
In either area (frontend / backend) the complexity depends entirely on the problem that you are trying to solve and I am sure there are plenty of FE problems that require a high level of expertise to solve elegantly.