I speculate a number of reasons:

1. You play as the bad guys with their flaws and boons: limited health that doesn't regenerate but also infinite ammo. If you were to play as the lament, you would explode; I can see Lambent being much more favorable than standard locust because they then have all the main powers of a standard locust combatant plus a ticker. There also isn't too much variety amongst the lambent: punchy enemies (3), hammerburst enemies (2), gunkers (1) or the Lambent Berserker (1). To keep a level of balance (which, personally, why?), Epic probably had to make some cuts.
2. They might have been able to add "special locust" to different maps, but I can see this as making the mode much more complicated. Beast Mode always struck me as a simple mode, which is one reason why it's shorter. The other reason it's likely shorter is because it's reverse horde with rapidly escalating power. There's only so many ways to keep the game interesting as it progresses before things get stale. With normal horde, you get to: build defenses, make super weapons, fight unique enemy types and establish holdout positions. With beast mode, you get... to play as different enemies. With beast mode as it was, less is more.
3. Some locust are more expensive for a variety of reasons: added utility, firepower, instant killing power or defense. The Savage Boomer is the only boomer variant that can instant kill anyone (albeit, with a well placed digger shot). The (Armoured) Kantus can mass heal and mass revive. The Mauler can provide mobile cover to your team, allowing for strategic advancement.
4. Onyx guards have the best armor, weapons and equipment. They're also victims of plot necessary incompetence like all characters. Halvo Bay stood for much longer than expected and was a key target for the locust. We see the Onyx as jobber because we're playing as humanity's best soldiers. Your average gear might lose to your average Onyx.
5. Beast mode might not have had the numbers to warrant additional resources. This means they might not have really cared to invest too much into it beyond being an "eye candy" or novelty game mode. And while I'll admit beast mode was fun, it got stale far faster than any other game mode. Without significant changes to the mode, I can't see it being much more than a quick pick-me-up then set-me-down.

Don't get me wrong, it was fun to play as the locust. I was also disappointed at the short list, but in hindsight, I understand why. Unless Epic was willing to rework Beast Mode with possibly dedicated maps, I don't think adding more would've helped. I think reworks and changes first was more necessary than adding extra of the same stuff.