Technically, the RAS SACU presets could be considered "new" units. I don't know if those predate or postdate the HQ/t3 maa.
I don't think it's so much a formal policy against adding units, as it is that there are big barriers to adding units
1 - they would need a properly-animated 3D model
2 - they need to not ruin game balance. if they don't improve the balance, why add them? So that takes a lot of testing. The way people use units can evolve over time and as people discover how to use units better, they need to be rebalanced again.
3 - you might need to add 1 for each faction just to maintain balance, so quadruple that workload. And that means you need 3D modelers who can make designs that are consistent with all 4 faction styles.
4 - it could affect or break a lot of mods and AI projects. If you add new features to the game for the new units, that can break things. So you may end up creating a lot of work for a lot of people.
5 - Players coming from outside of FAF then have more to learn in order to play the game.
The disruption to the game is generally a negative thing. You need a very compelling situation to justify adding a unit. I think people in general don't want to work on making new units for the game if the units probably won't be used. Even an unpopular map will get some play but if you make a new unit, there's like a 99.5% chance it will never be used in a serious game. At best it gets added to a modpack. So people don't even try.
If there was an active modding community that made 10 new professional-looking units every month that were carefully considered to fit within the game balance, we probably would see more change-up in the unit lineups with units being added/removed, models replaced with better-looking models, etc. But where would we get the kind of energy necessary for that? It might take 100 people cooperating to crank out 10 units a month. Imagine if there was an active modpack scene where the new units in the modpack were actually well-balanced for FAF games. If that existed, over time there would probably be a lot more crossover between the modpacks and the official FAF unit list.
Also, the mindset for a competitive RTS gamer generally should be learning how to work with the units you got. You can try to figure out ways to use units more creatively, but you don't get to change the game to bend it to your desires. You have to work with the game's mechanics/balance to try to improve yourself. You don't change the game, you blame yourself, watching some replays, practice, and try again. So most high-rated players aren't even interested in spending their time to expand the game, or at least they won't develop that interest until they've played 4000+ ranked games. So the people making the units would largely be guys with about 600 ladder rating. Which means they probably don't have great instincts in terms of making units that fit within the game's overall balance.