SBC 265.283/327/350 and BBC 396/402/427 were all factory internal balanced because that is the better way to do it and it was economical. SBC 400 and BBC 454 were factory external balanced because it was not economical to do it internally and external was. The natural progression of engine family increases in cubic inch was done more by stroke than bore. At a certain point, the cranks got so big(stroke) it was just cheaper to go external.
I imagine other brands of engines are done that way for same reasons. JMO.
Cheaper aftermarket crankshafts may be external balanced for economy reasons also. Better aftermarket crankshafts are internal balanced for best performance.