That's surprising. Making a series is expensive, you would expect it the other way around, as there has to be a certainty that the show will actually be broadcast. But I suppose the negotiations are often on the basis of a pilot?It's the production company that first negotiates with a distributor for broadcast rights not the other way around.
So, if I get it right, the production company initiates the whole thing, buys the rights to the book, makes the pilot and they are the ones that try to sell it to (different) broadcasters everywhere.
That would explain why there are sometimes pilots of series that exist, but are never broadcast or seen anywhere. In that case, they just couldn't sell it and the series remains unmade.