The US Ambassadors to the ASEAN countries rotate every three years or so. For the most part, with the exception of Singapore, they tend to be experienced career foreign service officers. A new class is making its way into the region. They a relatively young group with strong resumes and solid experience.
While the idea of combining the jobs of US Ambassador to ASEAN with Special Envoy for Burma offers a certain bureaucratic efficiency, pursing this option would undercut the ability of even the most experienced diplomat to deliver on each portfolio.