Well, the most simplistic explanation is me knowing there is something better and within reach!
I have listened extensively (not at home, mind you) to the KEF R series (both old and new versions) and really like the way they sound. To not blindly go all in, I compared the R series to the Q side-by-side at the dealer.
That being said, I really, really like the new Q series and feel they are much closer to the old R series than dealers were willing to admit (before the new R series came out). What bothered me about the Q750 were a few things: The cabinets were not all that well damped, were almost lively and I could "hear" them. I know it is an issue of cost savings (or purposeful - some speaker designers like a lively cabinet and "tune" it) but if I stacked, say, 20 pounds of books on top of the speakers, they tightened up a bit. That is really neat, but I am really not a fan of such DIY solutions. The other thing is the "midrange" driver that houses the tweeter runs full range and the midrange is not as clean as it could be. The R series has dedicated midrange drivers in a 3-way configuration, not "2 1/2" way, as the Q series is described. Listening to a Q and R series speaker side-by-side make this rather obvious. Lastly, as stated by someone else, the finish on the R series is SO much nicer. The p-lam on the Q is fine (and I like that it is not pretending to be wood) but it certainly is not real wood veneer or gloss.