We can't know why Boeing did what they did unless they tell us and given the legal implications of saying too much there's more noise than signal. I can speculate with the rest on an answer with the rest of them.
The 737MAX MCAS was meant to provide a certain "feel" on the control yoke to the pilot when making specific maneuvers. MCAS would be expected to make minor corrections to the trim, do so rarely, and any failure was expected to look like a runaway trim or other failure that pilots were already trained to address in the 737NG. This was not considered a flight critical function and so used only one sensor.
On the KC-46 the purpose of MCAS was to mitigate the shifting center of gravity as fuel was being delivered to other aircraft. MCAS would be in near constant operation, it had what was believed to be far greater control of how the aircraft performed, and so used two AoA sensors for redundancy of input.
As I recall the decision to use only one AoA sensor was not to reduce workload on the engineering and testing but to reduce workload on the computers in the aircraft, computers that already had considerable workload for their capability. One AoA sensor meant not needing to take the extra computing power to check one input against the other.
I wish I could recall where I read this but someone that claimed to know what he was talking about said that it would have been possible to not only compare one AoA sensor against the other but to compute the AoA from other sensors to determine which sensor was giving good data if there was a disagreement. This is in effect a "virtual" AoA sensor so that there can be a vote of 2 out of 3 on what is the correct AoA. Additionally this was supposedly possible without exceeding the computing power of the 737MAX onboard computers. Assuming this was possible that would have meant additional work to create this software and validate it was safe. Given Boeing had a tight schedule, and MCAS was not considered safety critical by the FAA, this extra work was not done.
While the AoA sensors are on all 737MAX aircraft not all have this information available to the crew. The crew would only be told of an AoA sensor mismatch if the airline purchased the option for AoA indicators on the flight deck. This was apparently not a popular option, few 737MAX aircraft had AoA indicators. Again, there are two AoA sensors on all the 737MAX aircraft but not all aircraft had AoA indicators for the crew, MCAS had this information but the crew might not.
Airbus apparently views AoA indication differently. They put three AoA sensors on their aircraft so that single sensor failures can be detected and corrected. Again, there is a claim that Boeing could correct for a single AoA sensor failure by computing AoA from other sensor inputs but they chose not to. It's possible they chose not to out of legal reasons as much as practical and technical reasons. If they demonstrated the ability to calculate AoA in order to detect and correct for a single AoA sensor input failure then that could open up another investigation on why this was not done before.
Boeing may have to dig in their heels on maintaining that AoA is "nice to know" rather than vital to controlling the aircraft so as to not attract more lawyers. I don't know how important knowing AoA is to a pilot as I am not a pilot. I expect this to be debated for some time.