I think one of the main reasons is that Disneyland is/was a smaller theme park in comparison to WDW and even smaller before DCA so there are less visitors that go and plan a stay in advance. I believe WDW has more international visitors too.
I think i also read somewhere that a lot of DL visitors are from California or nearby states and practically all AP holders come on a whim so most here would probably not use it or maybe abuse it by reserving even when they do not go
I've never been to WDW but I believe you are not able to just cross the park by foot unlike DL and DCA (in a way DL and DCA is just one big park if you have an AP or park hopper) so fastpass is less necessary because you can just jump back and forth with little effort compared to their Florida brethren.
It's definitely a convenience I would like to have but not if it is going to be a big mess and definitely not if I have to pay extra.
These are all guesses of course, chances are the biggest reason is that they do not want to invest too much on it and only found it viable lately and wants it perfected, I think the DL app only released last year too?