The Doogs are the workhorses of the Crux. They're earnest, hard-working beings with a deep, abiding trust in all creatures. They are loyal to a fault. Their loyalty often looks like stupidity to more cynical races, but they're not stupid, merely unsophisticated and more than a little naive. They have the kind of compassion and sweetness usually associated with small children, which makes them look somewhat slow. But they're not slow; they just sincerely believe what they're told. Their ships reflect their nature: They're builders and defender's, not warriors.
The Doogs don't serve the Crux voluntarily. They were enslaved in an elaborate scheme that took advantage of their guileless nature. The Ploxis tricked the Doogs into working for the Crux by secretly directing K'tang to attack the Doogs' richest worlds. The Ploxis offered to broker a "truce" between the K'tang and the Doogs, but not before the K'tang destroyed the Doogs' food supplies.
To fend off starvation, the Doogs purchased emergency rations from the Ploxis at massively inflated prices. When the Doogs were unable to pay, the Ploxis loaned them the food at outrageous levels of interest. The Doogs must work for the Crux until this debt is paid.
This will never happen, however: The Ploxis have orchestrated an elaborate "company store" price-fixing scam. The longer the Doogs work for the Crux, the deeper they become. The Doogs must buy all their colonising materials from the Crux, who charge them more than they pay for the completed work. The Ploxis call this a "free market," and have convinced the Doogs that it's fair, even though they forbid the Doogs to buy materials from any other source.
The Doogs not only accumulate debt faster than they pay it off, but the harder they work, the faster they accumulate debt. They also must pay annual "protection dues" to be members of the Crux, protected from Crux enemies. Adult Doogs owe not only a lifetime of labour, but the labour of their offspring. Soon they'll owe the labour of their grandchildren.
And so on.
The Doogs are sad about their servitude, but their innate sense of decency keeps them from rebelling.