The problem is not to be dominant, it is that there was no money to be made in this type of website. These sites come and go, it is very difficult to monetize them.
And there are very few things like Wikipedia that both have the benefit of being dominant and not monetized. Are there any other examples at all?
I'm willing to incur some hosting costs for a beta period but I won't fund the service for long without revenue if the hosting costs are prohibitive as I'm not really interested in selling it later for someone else to monetize with ads.
Delicious once sold to yahoo! For 15 million, bought for 35k.
https://news.ycombinator.com/item?id=14463102
Delicious spending twice as much as pinboard in running costs in 2017.
There are some detailed blog posts from the time about running costs somewhere.