You're looking at it through internet-era glasses. We're talking late 70s to early 80s tech and prices. Equipment was
expensive. Clients were charged by the minute. Without city-level populations, the numbers didn't pan out. If the prospects were brighter, companies would have been more serious about converging on a standard.
A late 80s / early 90s BBS is no comparison. Cost-per-everything in computers had plummeted by then--even kids could host a BBS with the family computer.
Per wiki on Prestel (i know it's videotex, but the article covered both):
Hosting Costs:
> In 1985, British Telecom estimated that for an IP using a typical minicomputer (such as the PDP-11) located 100 km from London and handling up to 10 users simultaneously at peak times, the one-off software set-up cost would be at least £16,000, communication costs would range from £4,280 to £5,550 a year (depending on the type of connection), and Prestel usage would cost £8,600 a year.[82]: 4
Usage Costs:
> At the launch of the commercial service in September 1979, and in addition to phone charges, users were charged 3p per minute online to Prestel from 8 am to 6 pm Monday to Friday, and 3p for three minutes at other times. Installing a phone jack-socket cost £13, with a quarterly rental of 50p. Business users paid an additional standing charge (i.e., a flat charge regardless of usage) of £12 per quarter.[23]
> By October 1982, the online usage charge had risen to 5p per minute (8 am to 6 pm Monday to Friday and also 8 am to 1 pm on Saturdays, free at other times), the business standing charge to £15 per quarter, residential users now paid £5 per quarter, and jack installation cost "from £15", with a 15p quarterly rental fee.[24]: 2
Content Distribution Costs:
> A main IP rented pages from the Post Office (initially) or British Telecom (later), and controlled a three-digit master-page in the database. In 1982, this cost an annual £5,500 for a basic package,[24]: 1 equivalent to around £29,000 in 2021.[80]