(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.
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...
Another good reason IMO (3) another implementation of the conversion may be a useful reference/point of comparison