The widescreen is
B5's original aspect ratio. Straczynski had it framed for widescreen because he figured that HDTVs would be A Thing in the not terribly distant future.
The CGI wasn't rendered in that high of a resolution because they were using Amigas, and they figured it would be easier to re-render the CGI in whatever higher resolution became the standard down the line. No one expected the master files to be lost.