I’ve heard its because of 3 wires in 3 phase AC instead of 2 in DC. However I would think the more important factor is that AC would have higher peak voltage than DC which would mean longer arms and insulators on the towers. The size of the tower is relevant as this is one of two cost savings that make hvdc profitable at long enough distances. The other being power factor (phase shift/ reactance) losses from capacitance in AC.
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Insulators are creepage limited, not clearance (assuming correctly designed), so they are limited by RMS voltage, not peak. Capacitance in AC overhead lines are negligible at fundamental frequency whereas inductance isn’t so the resulting phase shift is not capacitive but inductive. – winny May 07 '22 at 22:27
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Limited by the size of the tower, for a 1000+km transmission line, the inductance is close to unmanageable. The inductance among lines and also with ground is the motivation of DC. – user3528438 May 08 '22 at 03:13