Sort of funny, in a way. 3600 hp locomotives had been offered as early as 1966, but by the mid 1970s the railroads had by and large decided that they wanted 3000 hp units (in six-axle size: with a few exceptions, most of them preferred 2000/2250 hp on four axles), and kept to this until the early 1980s. Quite suddenly, they decided that 3000 hp wasn't enough to be worth buying: GE catalogued C32-8 and B30-8, but the only production units sold were one order of the 4-axle version to NS.
I suspect the radical improvements in wheelslip control made at the end of the 1970s had something to do with it: locomotives of the SD40/SD40-2 generation were dispatched at around 15% adhesion (i.e. it was figured that their worst slogging would be at speeds where they would exert a tractive force of about 15% of their own weights). 50-series EMD and late Dash-7 GE could dependably work at 24% adhesion. All of a sudden 3000 hp on six axles seems like not enough power to be worth spending money on.