Because there were minor differences in hardware (at least the supported frequency bands, but maybe more, e.g. firmware details / configuration) in the country / region specific variants of Xperia models. At worst there could be slightly diverging partition layouts, that is why I would not exclude the possibility of flashing to fail (or booting after flashing).
But I see that you have far more experience than I do specifically with the Xperia variants for NTT DoCoMo JP and that you experienced with the models you used that the only difference apparently was the provider lock. Still I wonder then, why other provider locked Xperias (e.g. by Vodafone) did not receive a different variant name? One may assume that there are additional differences. Besides the single SIM and dual SIM variants requiring different flashing images despite their > 99% similarity (as @attah already pointed out) Jolla also stated that the Xperia 10 IV and V regional variants 54 and 72 (i.e. XQ-CC54 / XQ-DC54 vs. XQ-CC72 / XQ-DC72) require different images.