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.
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.