When you say XM-L, do you mean MP-L? XM-L's are single die.
Not XM-L easy whites, they even come in the U3 bin IIRC, 4 XP-E dies it looks like (EZ-1000 or EZ-900 IIRC)
First I'll say that I'm not an electronics guy, but my understanding is that it is easier to design a driver for a circuit of four die driven in series (high voltage, low current) than one big die like the standard XM-L (low voltage, high current) in particular for 110VAC to DC. Perhaps not so much "easier" as in difficulty but as in more robust over time.
Youre on the right track, when it comes to semiconductor stuff, for low voltage stuff (<50-60v ish) voltage handling capability is cheap and current handling capability is more pricy, in both efficiency losses, and part costs. Not only that, there already exists 350mA, 700mA, 1000mA, 1500mA constant current LED drivers with a universal AC input(11 on mouser, 39 on digikey atm) . For someone making a light fixture, an off the shelf solution for a ballast would be preferable to having to design one. Not only that, if the ballast has various certification (CE, UL, etc... ) and the output is <60v, then all the other wiring is low voltage per NEC, and in the US at least, that would make safety certification cheaper and easier.
For bulk, flood lighting, I doubt pricy, high powered, large die LEDs will gain popularity. But they do have their purpose, as we know.