I've seen this too many times. A new forum on some subject, with thirty or more sub-fora, great most of them with 0 posts, some with 2-3, one "welcome topic" running greetings from some 40 users the site didn't retain, as they left simply because too little was going on. Some questions answered best to the crew's ability, because there was nobody else when they were posted, some posts inviting discussion but hardly keeping it up, and maybe one or two never-ending forum games which the few users get quickly bored of.
Other sites on the topic don't quite approve of advertising competition, and even if my site is technically superior, the users stay elsewhere (even if the competing site is abysmal) simply because "everyone is there".
When I moderated a site which had a country-wide monopoly on the subject, I saw dozens of attempted "competitors" die that way. Only when a major schism happened at a time - a conflict between the site owner and one of more prominent user, that user managed to pull a sufficient amount of users to the site he started, to kick off and create a viable competition. And while both sites still thrive, years later, there's a lot of bad blood and hostility between the two communities.
How can one kick-start a new "topical" site, getting enough users in a short period of time, so that the community can keep propelling its own growth instead of dying out from solitude - other than starting a war with the established competition?