You could always handwave it away as the Seven Great Youma not really being fully awakened or properly prepared. Unlike the other Shitennou, Zoisite didn't have the capacity to work with these Youma beforehand as part of an evil scheme. Whereas all of the other Youma in the series were directly under the control and working for their masters and were either biding their time or undercover, these seven were simply manifested from slumbering souls. Given that they only said their name when they spoke, this indicates their awakening was not complete or simply botched. Compounding this is that Zoisite prioritized the retrieval of the Rainbow Crystals, so not only is there no plan beforehand, but the boss can't even manage the subordinate mid-fight beyond a couple of basic commands.
Of course, the real reason these villains are weaker is two-fold. One, with an increasing cast and with multiple people having different agendas in that arc, the fight scenes had to be simplified. Two, since Sailor Moon has to heal these people and some of these people are recurring characters, those in charge can't have her beat up on them too badly; deep down they still had "human hearts."
It would have been an interesting complication if once a Great Youma awakened, then it would prove to be too powerful to be a minion and would go against Zoisite (for being an "unworthy" champion of the Dark Kingdom in a bit of classic arrogance) and our heroes.