In my experience pandoc is not better than TeXmacs in converting LaTeX. The problem, as vdhoeven stressed, is that LaTeX is not a document format which can be converted reliably in something else. Usually I just convert to LaTeX the last version of the document, e.g. before submitting to journal. In this way I always dispose of some alternative storage formats: TeXmacs, PDF, LaTeX and sometimes HTML if needed. The LaTeX produced by TeXmacs uses few packages, which also means that it can be easily maintained.
(1) You may want to work with other formats. (2) You may not trust TeXmacs to be around in the future or to be installed on the machines you use. The ability to convert using Pandoc would not prevent anyone from using the existing conversions.
Another good reason IMO (3) another implementation of the conversion may be a useful reference/point of comparison
I agree that having more converters would be nice. But which formats are most urgent? A high school teacher would argue that we should support conversions for Word. Programmers would stress that we support Markdown or Pandoc. GNU hackers would ask us to support Texinfo. Any help is welcome...