It's not so easy to say what is so compelling about it. Certainly, US series and those from HBO in particular (e.g. Sopranos) have high production values and equally high standards for scripts and acting. But they also score because somehow US 'tec or crime dramas are just more exciting than ours. Perhaps crime is more interesting in the US. More likely, they have cottoned on to the idea that there is more to this sort of programming than leading the audience into a guessing game of whodunnit. (Compare our ploddy but otherwise OK output such as Morse or Waking the dead...) The Wire never hides whodunnit. What's more interesting is howdunnit or whydunnit, and the careful and gradual exploration of character, circumstance and the pressures of life in a gang (whether on the street or in a police team). Only rarely does it descend into moralising or the annoying habit some shows have of suddenly making a character voice some unlikely profundity (as when D'Angelo, about to rat on his family business, starts describing how he was more free when in prison than under the control of his uncles.) Still, just imagine the writer of the BBC's New Tricks coming up with this, from David Simon who created The Wire:
The second season of The Wire, centered around Baltimore's dying port unions, is a meditation on the death of work and the betrayal of the American working class, it is a deliberate argument that unencumbered capitalism is not a substitute for social policy, that on its own, without a social compact, raw capitalism is destined to serve the few at the expense of the many.