The steepness of the coned material in the center of the kettle is dependent upon the 'shear strength' of the material. In the case of the ground hop matter in pelletized hops, the shear strength is nearly zero since that cone is composed of hydrated proteins and vegetative materials. Essentially, its goose crap.
If you use whole hops, then the cone can take on more shape because there is mechanical, 'particle to particle' contact between the individual hop cones. That mechanical contact increases the shear strength of the mass and allows it to 'cone' better. One of my old brewing buddies pointed out that a good technique for improving the coning of the hops and trub was to include at least a small proportion of whole or plug hops in the hop bill to provide that strength to hold the cone together. I prefer pellet hops, but using 100% pellets hops will leave you with a 'flatter' cone nearly every time.