TL;DR: It’s the only symphonic march. Loads of brass chord progressions.
Because Jerry Goldsmith did a great orchestral arrangement riffing on the old sci-fi 60s style theme. He made it very grand with Symphonic March style and the layered horn/trumpet progressions. It is a revision of what he had previously written for Star Trek: The Motion Picture, which might also be part of why it’s a March. So, if we look at the styles of a bunch of the the themes:
TOS: fast 60s vocal arrangement
TNG: symphonic march with cascading brass/french horn progressions
DS9: Slower non-march style of symphonic theme
VOY: Slower grand/inspiring symphonic theme
ENT: Well, it’s a pop song.
DISC: Very moody/pensive slow start, growing slowly to a note of inspiration.
PIC: Similar to DISC with a very slow/moody start. Lots of melancholy layering.
SNW: Almost between a march and mini overture in feel, but without all those cascading horn progressions that TNG had.