I can't speculate, but there are significant differences in the way they have historically viewed and treated mental health conditions. Also, there are significant differences in the support provided to their troops and in the duration of their deployments, for example, and the frequency of their duration. There are just too many variables for us to pin it down to particular things.
But there are many differences in the way the U.S. runs its armed forces, and in particular how it deals with suicide or mental health or stigma, that make it unfair to try to make direct comparisons.