Bargaining power of customers (the members)The 'switching cost' is to high. If a member leaves the WTS, he/she will suffer from being disfellowshiped and loose friends and family.
Not a very clever marketing strategy really. "Buy our brand or we'll make your life miserable". Bound to alienate a lot of customers.
Bargaining power of suppliers (the apostates and other denominations)
The apostates don't really have a tangible product to substitute the WTS product. All they can do is try to prove the WTS is wrong, that's about it.
This assumes that people have to have some organised religion, like they have to have some brand of toilet paper. But they don't. You don't need to offer a better brand of cigarette when you're trying to convince someone to quit smoking.
New entrants into the market
There are no new religions around to match the WTS product.
See above.
Substitute products and services
Substitutes products (hellfire, immortality of the soul, trinity). JWs have learned to reject these doctrines. It is hard to deprogram them to embrace it again.
It doesn't automatically follow that leaving JWs means embracing these beliefs.
Existing competitors
Roman Catholic Church, Mormons, Seventh Day Adventists, Salvation Army. JWs are programmed to view them as false religions. It is hard to deprogram them to embrace it again.
There are many, many other competitors besides these. Along with the dozens of brands of Christianity, there's all the Eastern religions, the revived earth-based religions, and of course, there's the start-your-own option. It's like the cookie section at the grocery store, you're bound to find something you like. And if you don't, you can go home and make your own.
OTHER FACTORS
Marketing
Zealous evangelizing of the JWs will only enable them to recuit more members as time goes by.
This is optimistic. 8000 cold calls leads to 1 bible study. In all my childhood as a JW, 15 years, my mother who went out in service every saturday achieved one bible study, and that was with a woman who was mentally disabled, abused by her husband and so desperately lonely that she'd have agreed to anything in return for a little kindness. She was never baptised, and when we moved, she stopped studying, since nobody else in the congregation wanted anything to do with her. My mother, by the way, is an extremely charming and persuasive woman, and if she had only taken up selling real estate or cars she'd probably have done quite well. But it's hard to sell a product people have little use for.
Medicine on their side
For the blood issue, alternatives are coming up and the field of medicine is now buying the idea of bloodless surgery.
Catchy slogan: "Buy our product and you won't have to watch your children die after an accident."
Past mistakes of interpretation
The JW GB are now vocal in their admission of the past mistakes they did. After all, they are only humans.
I don't know how vocal they are. Plenty of people still think the UN involvement is a lie, don't know who Bill Bowen and Barb Anderson are or what they were disfellowshipped for, don't know that blood transfusions have been allowed in Bulgaria for some time, don't know that there is a grave injustice in the treatment of party cards in Malawi compared to Mexico, don't know about the compromising letter that the German JW leadership wrote to the Nazi party during Hitler's rise to power. Have there been any apologies to those who's loved ones died before there was new light on transplants? To those who had to go to jail before there was new light on community service during the draft? To those who were disfellowshipped for being raped? To those sexually abused children whose elders told them to keep quiet about it? I haven't heard about it if there were such apologies.
Management styleAs the management is changing and getting younger in terms of age, old 'beaucratic traditional culture' is slowly changing to a more relax management style.
That's got to help. But when you come down to it the JW product is the promise of protection when Armageddon comes, which includes a sense of superiority because god loves them and nobody else, the conviction that everyone who disagrees with them will die soon, and lots and lots of rules to follow. That's just exactly what suits some people, but I don't think there will ever be a mass market for it.