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.
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.
That is the greatest flaw of the Primarchs. Despite their immense superhuman abilities, they are ultimately human, with all of the inherited strengths and flaws that we all have. That was why Horus and the others fell. Their flaws and inner demons got the better of them and led them down the path of damnation, like so many others.
Any Primarch created by the Imperium now, would be human, just like the other Primarchs, which would mean that there would be the risk of them falling as well, becoming more threats to them. The High Lords would never take that risk, despite the benefits it would yield for them.