Not sure if anyone mentioned this or not but by the time I purchased Traveller in 1979/1980 ish there was a lot of stuff published for fantasy games, and the one of the biggest differences to me is the lack of illustrations in those LBB.
D&D had really established a euro centric medieval style game with Tolkien inspired elements. You could page through the books looking at the illustrations and get a good feel for the settings.
The lack of illustrations, and more importantly a set of standardized norms for a sci-fi setting was a big detriment to running the game. The problem with the wide range of technology that I could imagine was that every single activity that the players undertook took explanation. Was this Star Trek, Star Wars, etc. Were there transporters, photon torpedoes, fighters, laser swords ? What did the bridge of the starship look like ? What are the procedures to land a ship ? What do the different types of weapons and armour look like ? What do the star ships look like ?
The big advantage that Star Wars rpg and Star Trek rpg have is that to understand the look and feel of the setting takes one viewing of a movie or a couple of episodes of a TV show. Those games are also lavishly illustrated.
Any time a Traveller book was published with an actual illustration was a big deal to me. I can still remember most of those illustrations.
To me it felt like the GM had to literally invent every aspect of everything the players were going to do. A picture is worth a thousand words. Without those illustrations the GM had to come up with those thousands of words of description for the most mundane activity.