I'll get right to it. There is no reason why Apple cannot provide mouse support on the iPad. They can and they should. It is certainly not an issue as far as trying to figure out the best US experience or the coding to support it. It is likely Apple had mouse support for the iPad way, way, way back then but just decided that touch was what they wanted to go with. It's likely that they've figured out the best way to implement a mouse on the iPad and even figured out the best and unique ways to use it. And they just chose not to support it.
If I had to take a guess, it is an issue of philosphy. Just like Apple will not support the Apple Pencil or touch on the Mac, it's the same as far as mouse support on the iPad. After all, Apple is placing the iPad as potential replacements for tens of millions of PC and Mac users, shouldn't they at least afford us some sort of a transitional UI experience?
Let's just say if that I can use a mouse with my iPad, I would be able infinitely more likely to use it as a productivity machine instead of using it only to satisfys my blood lust in Clash of Clans or blasting away aliens and zombies. Sure, I'm using my iPad to do work. But I have artificially limited to what I want to do with it.
Which brings me to this point. Perhaps, Apple has seen data that shows how and what people are using the iPad for as far as work is concerned and having a mouse isn't necessary. But it could be that others are also limiting themselves only types of work and apps that they can more easily use on the iPad becaue there is no mouse support. And therefore, Apple's view on this issue is skewed.
Then there is the issue of competition. Both of Apple's major competitors in the mobile and PC markets, Google and Microsoft, both support more than one way for input - most glaring is that their OS support the mouse. Forget the low-end of the mobile pool where Apple does not like to swim in but in the high-end, Microsoft has actually shown "courage" by coming out with a line of Surface products that are really a threat to Apple. Sure, they're no Macs or iPads but they could be good enough for many of Apple's customers. And yeah, they look nice too. (This is a jab at Apple's VP of marketing, Phil Schiller, who famously said it took courage for Apple to remove the traditional 3.5 mm headphone jack. Apparently, many agree it was not really courage - Mashable, Techcrunch).
So, removing a 3.5 mm headphone jack doesn't really bring up courage in my mind but if it's simple philosophical stubbornness or bad data that has kept Apple from support the mouse on the iPad or the iOS devices in general, then perhaps, Apple could admit it and start supporting the mouse in iOS 11. That would be kinda courageous.