What are the reasons? Yes shunning is likely to be a factor. What else? The high level of commitment required. The sense of mission preaching gives for some. The crazy state of the world right now in view of JW end-time teaching. Attraction to a definite black/white set of beliefs and rules in contrast with changing ethical standards in society. Everything being spoon fed and not having to think too much might appeal to some. I don’t know what else. That’s just some explanations I come up with.
The difference in the fate of mainstream churches compared with JWs is extreme. Takes these two graphs. The first shows the decline of members of the Church of Scotland since 1957 where membership has dropped to about a fifth of the peak. The second shows the increase JW publishers in the UK over the same period.
There are single churches in other denominations that are growing, but I don’t think there’s any other denomination over 100,000 members still growing in the UK as JWs are. But I’d be interested to proved wrong if anyone has examples. Perhaps most likely candidates are Pentecostal churches but even these have declined in recent years.