Because wiring the motors in series has a sometimes undesirable side effect. The motors will exhibit a differential action... if one truck loses traction and slips, the other truck/motor stops. The stopped motor has resistance approaching zero, and the slipping motor gets twice the normal voltage, approximately doubling it's wheel speed.
This would be difficult to explain to buyers.
That said, later K-Line diesels had a DPDT switch on the bottom to switch between parallel & series.
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