It’s partly that JMS favoured stage actors. Partly that he grabbed a number of his favourite actors from “Murder She Wrote”. Honestly brilliant, taking a couple of murderers of the week and giving them lead roles in a show.

There's more to this line of thought. The pilot movie isn't space opera so much as it is a murder mystery in space, and it's not the only episode like that. The security chief, Garibaldi, is a hard-bitten alcoholic detective from film noir (and there are plenty of film-noir-like visual elements: fans, shadows, run-down industrial sectors unimaginable in Star Trek), and he plays a much larger role solving crimes than do his counterparts in other science fiction shows. The writing style, a long arc that drops clues up front to pay off in revelations down the line, also leans heavily on mystery writing.