There are a number of reasons why this is more challenging. I suspect the primary difference is getting the rockets actually to Wallops Island. The rockets are made in New Zealand, not far from their launch complex. Transporting them there is quite easy. Transporting them across the world is more challenging, to say the least.
Wallops Island is actually pretty difficult to get large payloads to, such as the first stage. The final trip there has to be done by street transportation. The area is quite rural. One example, some of the traffic lights have been moved to allow large rocket stages to get through such as the Antares rocket and Cygnus spacecraft, but still, it is challenging. There are likely limits to how frequently the rocket stages can be brought in. This doesn't even include the shipping, which may have limits to the numbers of rockets that can be brought in at a time.
The Electron pad at Wallops Island shares much of the infrastructure with the Antares rocket. For instance, there is only a single water tower between the two launch pads. I believe there are other pieces of infrastructure that are shared between the two as well, but I'm not aware of them.
Other issues could include difficulty getting fuel there (unlikely), or range availability (Likely). Resetting a range takes some time, and as Wallops Island has never had a particularly high launch cadence, even 12 times per year is about 3x their usual capacity for orbital rockets. Launching rockets there will require rerouting air traffic from New York to Miami, and likely quite a few other flights as well.
Lastly there is preparing the payloads. The industry standard is to give each company 30 days to get their satellites unpacked and prepared to fly. Likely they have multiple areas in New Zealand to get the satellites ready, at Wallops they only have one.
I suspect that launching from Wallops Island will be more expensive than from New Zealand, and as a result, they figured there won't be the demand to justify getting more rockets. Still, if the demand is there I'm sure they can increase the infrastructure to support more launches, provided they can get the range cleared.