One aspect could be - If you want a narrow cabinet, you must use smaller diameter midrange. If you use smaller midrange, you don't have enough cone surface area and distortion goes up at low frequencies. If distortion goes up at lows, you must move the crossover point higher.
Other aspect could be - 4" midrange crossed at 250Hz with 4 woofers per cabinet is a serious workout for that little fellow. Since at Kef they had to downsize the midrange voice coil, it now has less power than it's larger brothers from R and Blade/Ref series (they've reduced voice coil diameter for the midrange to get more cone surface area for it, but now they had to use 19mm tweeter since larger 25mm one wont fit). That probably pushed the mid-tweeter crossover higher aswell.
They could have used Nd magnet behind the midrange which would add more SPL capability for the small cone but that would probably drove the price up significantly.
There could be more but one must understand that in loudspeaker design you are dealing with chain reactions rather than isolated problems/goals so engineers with "tunnel vision" are less desirable