The reason is simple: They have access to the most interesting toys (tech and mechanics) from the get-go:
- Slaves
- Cloaking
- Energy Torpedo
- Cruisers
Plus they boost the coolest ship designs, and especially their capital ships are available in numerous designs and they are all very sleek and intimidating

If I had to list the others and why they fell shorter then the mighty Gremak:
2. Ashdar Colonials: I just love fighters + they can really crank up production right ffrom the start
3. Orthin: Cool design, unique ship class with the sniper and they realy break through the physics research tree making for an interesting tech oriented gameplay
4. Humans: They feel closest to home, and their smaller ship designs really hit the ball home. Not that fond for the Marine art (too clean for me) and the unique starting position quickly disappears after the first colony. I always felt there would be more to them living on ships. Boarding mechanic is fun but is just as viable with other races (usually produce a dedicated boarding light cruiser with Gremak

5. Ashdar Imperials: Same as Colonials, but slower production. Stargate is a cool mechnaic though to get for their homeworld
6. Yoral: Lots of cool ship designs but nothing particularly else. I am also very much used to mostly battling these guys and thus their image is worn off a bit. Wish the others would put up a fight like them to balance things out
7. Phidi: Nice ship designs, but the others have more variety. They should have an interesting game mechanic concentrating on trade, diplomacy and mercs but the later two need to be extended for this race to be truly viable