Nexus seemed to be the one thing Phantom couldn't get on his own and for that, he needed the help of the Citadel. The blacksmiths were taking Phantom's genes and creating something completely new out of them, but actually, they were just helping those genes develop.
When Adam had to create his Nexus, Silvana had asked him to decide what kind of Nexus he wanted, to give his genes a specific purpose beforehand. Sure, sometimes things didn't go exactly according to plan, but either way, Phantom got roughly the Nexus he wanted.
However, once the Nexus was already created, and the Phantom was going from K1 to K2, the blacksmith's help was no longer needed, well... that wasn't true for most Phantoms going through evolution at that level.
To create an area where Nexus could evolve on its own like a living being required a tremendous amount of energy, considering it was a K1-Phantom.
