I think the 45 minutes reserve is required under 14 CFR part 91 (private ops), whereas the airline boys get away with just 30 minutes under 14 CFR part 121.
The difference is probably historical in origin. The airlines have dispatch departments and are also required to carry contingency fuel. They also have the concept of 're-clearance' available to them. These disciplines and techniques, coupled with the requirement to keep records, means that the regulations allow them to reduce the final reserve fuel to 30 minutes.
So I don't think the difference is necessarily due to 'international' v. 'domestic', but rather 'private' v. 'airline'.
For a more detailed and up-to-date explanation, refer to e-CFR
edited after re-reading the OP: I guess the longer sectors involved in 'International Ops' meant that the airlines needed the flexibility of a smaller final reserve fuel requirement at the planning stage. I suppose the rationale was partly that the longer sectors would have had more planning input and in-flight monitoring than the shorter domestic ones.