I guess one reason is that initial growth of online service providers (and especially payment processing) is often driven by sex-related use, e.g. sex work or art.

If the provider becomes "mainstream enough", these uses then get banned to satisfy investors, key customers and upstream services, driving the demand to the next startups.

PS: Or rather, were always banned from the start, but enforcement was lax until they were no longer an important part of the customer base.