Assumptions in a Moving Fluid

I saw this paper recently "On Integrals of the Hydrodynamic Equations That Correspond to Vortex Motions" by Hermann Von Helmholtz. It can be found here. In this research paper he discovered Toroidal Vortex. He did so because he did not make the assumptions that his predecessors made.
All other papers prior to his assumed Potential $V$ and a Constant Velocity Potential $phi$

However why was this assumption taken just for the ease of calculations? As I can see Turbulent Flow was well known before and there was no reason to make these assumptions of an ideal case so why did no one discover this result prior to him?