Have CA ever given a reason? I don't recall one.

I suspect that what lies behind it is a desire to have a campaign that covers the whole of a long period of history, running right through from the MTW "early-" to the "late+" period. CA want people to see their armies develop from basic spears etc to gunpowder units; and to see their operations expand from Europe to America. Basically, I think they want to get a little closer to the kind of rapid coverage of history you see in Civilization games for example. I think they are calculating that the greater variety introduced by this will appeal to the mass market.

Given the above, they want to accelerate the strategic gameplay. The mass market gamer won't want to commit months to a campaign. But if it proceeded at MTWs pace, then that is what would be required. I don't know about anyone else, but my MTW campaigns started in early and tended to peter out by the early high period, just about the time when I got feudal knights and high period units. I either became too dominant for it to be challenging or I got too bored (usually both). That pacing is contrary to idea I've suggested of trying to get a panoramic historical campaign. The implication is that CA are going to need fewer turns per year.

The problem then arises that with the RTW engine, they actually have more turns per year than in MTW. Raising the number of turns per year to more than MTW will make it apparent that armies are moving across the map far too slowly to be plausible. By any reasonable standards, they will crawl, as early armies do in Civ, for example.

So to fudge that problem, CA decided to end the fixed relation between years and turns.

That's my interpretation of why CA switched from years to turns.