Did tradition play a part? Certainly – but most traditions in schools actually have some common sense behind them at least originally.
Why did so many heads and governors of boys’ schools not opt for a non caning policy, on a similar scale to girls’ schools?
Because when it was tried – and it was tried from time to time in some schools – it generally failed to work. So the experiment was abandoned. When tried in girls schools, it succeeded much more often.
Behaviour among girls was no less or worse than boys
Untrue – contrary to the protestations of certain people, teachers have long understood that misbehaviour among girls is typically (though not universally) less prevalent than among boys. There is, frankly, virtually nothing in educational or psychological research to contradict this, however much it doesn’t match some people’s views of the world as they wish it was.