The proposal was to put a gun on a large armoured box that could move under its own power. To people with little experience of motor cars, that could sound implausible. Cars were still unfamiliar to many observers, and vehicles that bogged down hardly made a reassuring case for carrying armour. The objection concerned whether such a machine could move reliably at all.
Chapter · 4:42-5:29
Why a tank sounded like a crackpot scheme
Limited familiarity with motor cars and doubts about their reliability help explain scepticism about a mobile armoured gun platform.