It is a difficult question, but this is my theory.
Until the 1900s, it was the "bad guys" - the law breakers - who were celebrated rather than those who enforced it.
Take for example the thief Robin Hood:
Or the highwayman Dick Turpin:
Or even the underworld king Jonathan Wild:
In contrast, those charged with upholding the law were the villains. For example, the Sheriff of Nottingham:
Or the witchfinder Mathew Hopkins:
And the hanging Judge Jeffries:
Even the literary character Sherlock Holmes was a private operator, usually working independent of (or in spite of) the officials who enforced the law.
So, in view of the lack of a tradition of the law enforcers as "good guys", it is perhaps not a surprise that the first popular police screen characters were the slapstick Keystone Cops, and not anything more dignified:
Indeed, it was only the cinema and television age, with a fictional invention such as Dixon of Dock Green - following the American wild west sheriff and Elliot Ness traditions - that there appears any popular sense in England as a law enforcer as a good guy:
Why was this?
I think the answer lies in a sense of the law's legitimacy. Before the 1900s, legislation was associated with the "them" and not "us". Even though the sheriff of Nottingham, Hopkins, and Jeffries were on the side of law and order, and that Robin Hood, Turpin, and Wild were dishonest law breakers, the latter somehow had more legitimacy when the tales were told.
As such, perhaps the popularity of the criminal as folklore - or literary - hero is perhaps a good barometer of the popular sense of legitimacy of law itself.
All because the bad guys are often starkly without merit, that does not mean that those who wield the State's immense power get instant easy support.
The legitimacy of State power always has to be earned. The State does not, and should not, get it by default.