Because only the Emperor knew how to create the Primarchs. No other man, save perhaps Fabius, knows the secrets of the Primarchs and their creation. The Imperium can't make anymore. But I doubt they would even if they could considering that half fell and became among the most dangerous of their enemies.The primarchs were awsome and nigh unstopable. Why does the imperium not make more? It would mean that they would have an almighty general to comand them and they wouldn't lose and become extinct.
The Tau could genetically enhance themselves or members of their race, but they couldn't craft beings that would be anywhere near as potent or immense as a Primarch. Its heavily implied (almost outright stated) that the Primarchs were crafted using not only the unparalleled genetical technology that the Emperor was privy to, but also the raw energy of the Warp/Chaos (something which the Tau could never harness).Then they should make a tau primarch because none of them have ever fallen.
PS. It is possible for Tau to fall to or be corrupted by Chaos. Some undoubtedly have, others are rumoured to have (Farsight).because none of them have ever fallen.