To OP, here's my reasoning:
1) Biotics probably DID rule their areas of the galaxy. However, biotics doesn't exactly lend itself to technological advancements by itself. Advanced forms of it (seemingly stuff like enthrallment seen by Leviathan and domination seen by Ardat-Yakshi, etc) does show quite a lot of power, but I'd imagine it'd also lead to a much slower advancement compared to purely technological/synthetic advancement.
Basically, it'd lead biotics to rule a planet, but good luck controlling a system or galaxy with that mentality. You'd need to push foward tech advancement at a fast rate too.
2) Tech manipulates mass effect fields too. Heck, in itself, it does so at a larger scale that pretty much any biotic could. The tech powers we use even do special manipulations of the mass effect (it just looks different than how organic biotic users do it) that would require masters of manipulation to do otherwise.
Think of them more as two sides of the same coin. The innate biotic users may be impressive, but their effects are actually less impressive when we put it into a more advanced context. At that point, it becomes about tech powers that are programmed to do specific things that innate biotic users might never be able to do.
Consider it brute force vs manipulation. That sort of thing. Yeah, someone could toss an enemy, but if that enemy has ways to block those biotic fields, that biotic user is relatively impotent.
3) When it comes to enemy units, you need to understand that Shepard is put into SPECIFIC situations that fit the combat system. By and large, humans are NOT biotics, and it is considered a specialty/luxury/abnormality still, even as more humans are growing into it. Mercs? Honestly, we don't fight that many. Cerberus? They're a special group.
4)Using biotics uses a ton of energy off screen. It takes dedicated training in order to use a lot of it without your metabolism to slow you down significantly afterward. Sure, you can toss a bunch of enemies around, but then you'll be so out of energy that even a handful of others could easily take you down.
Biotics have the *potential* to be the most powerful concept of any person to have in the Mass Effect Universe. But this is also why it is 'lore nerfed' in several ways, and only niche concepts like the Leviathans and Asari have them, instead of just some larger species that campaigns as some super-soldier army.
Oh wait, we seem to have that too. The Prothans seem to be adept at biotics, and that probably helped them
I think the asari have the potential to rule the galaxy, but they prefer to be more the benevolent soft-controllers of it instead. There could be a lore reason written for this in a future game, for all we know.
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To non-biotics, especially in humanity (but also in turian and other societies to a degree), biotic users are almost another species, or evolution (for better or worse) of organic existence. Whoever uses it, does in fact have a superstition around them, that isn't very explored in the trilogy.
I'd imagine that a powerful and dedicated biotic could in fact rule a planet or more through their strength alone, but due to systems like the Citadel and Reapings, that doesn't happen. The closest we know is the Prothian Empire.
Biotics - Weaker regular potential than any other power, but the range of power could be immensely more than anything else
Tech - Higher regular potential than any other power, but the range of power might be capped off within this realm of existence (except for things like maybe the Crucible)
Transexistence - Highest regular potential of any other power (thus the Reapers, and apex philosophies of mixing organic and synthetic), but the range of power may be more stagnant once that point is reached
Main point with this? Yes, a particular biotic could rule the whole damn galaxy, but they would need the circumstances to be ripe for it, and biotics at large do not pose a enough of a threat unless the majority of them formed what amounts to those ME1 biotic cults x1000000 in strength.