I thought it was because full torque and full HP usually occur at different points along the RPM band. It looks like a good crossover point wherein the torque hands the job right over to the HP. Between the 2 of them, you've got a good linear source of power from 3k to 6.5k.
HP and torque will always cross at 5252. A small turbo running higher boost will achieve it's mass tq and then fall off quickly due to the compressor maxing out the volume of air it can push. A bigger turbo can run lower boost but push more air, and as such, the torque falloff will be far less
Think of a fire hose. The main attack lines are 2.5" and 3." When the engineer gets the pump set, the amount of pressure is a fixed value, however the volume of water being pushe is higher in the 3" hose vs the 2.5." The 2.5" hose is considerable lighter and more maneuverable and can usually be handled by one firefighter. The 3" hose is heavier, pushes more water which usually requires a 2-man operation