I don't know about running out of parts, but the implication certainly seemed to be that the non-US versions lacked the hardware. But if that were the case, why would it be disabled in US versions operating elsewhere? We need an example of a non-US MA2 operated in the US to answer this.
Perhaps legal reasons: if DJI didn't certify the ADS-B receiver yet in other countries that regulate such equipment, they might disable it there.