Don't forget luck. I mean sure, people hate the idea that forum/site success also partly depends on luck (due to some fallacy I can't name that causes people to attribute successes to their skills and failure to circumstances outside their control), but it's definitely a big factor nowadays.
And timing too I guess. A blog or forum about some hot new thing is always gonna be more successful than one about a dead or non existent niche. For instance, a site about the Xbox One or PS4 is going to do a lot better than one about the PS1, at least on average.
1. Effort - A blog will eventually pay off if you are doing things right however to get this blog to pay off you need to put a lot of effort into articles, eBooks, design ect to make people want to visit.
2. Advertising - Maybe another blog has done a better job at networking with other similar bloggers than you? There is always work to be done here and a blog cannot fail if you work hard enough to advertise it.
Good advertising will make a blog more popular than others. But it all depends on the way of advertising. There are hundreds of blogs with good contents but people hardly know of them. SERP also matters much in advertising.
If you need to advertise your blog to get visitors because you don't get indexed much by search engine, it just means that your content is not interesting and advertising a blog is definitly something I will not do since quality blogs get free visitors by search engines.
Eventually free advertising via forums and backlinks but even then, most traffic should come from search engines.
It took me over 3 years to get 300/400 UV daily on some of my blogs.
Also it's down to your blog niche, some niches are just quite impossible to get a high rank as all subjects are covered pretty quickly, some niches are just as easy as 123 to get ranked because not many people write things about those subjects.
If you want a little tip, try to look at keywords that are not mostly used for a niche and work on them
ie: xxx keyword get 45m searches a month and obviously it's hard to get on page 1
but
yyy and zzz keywords for same niche gets only 4m searches per month, then it's quiet obvious that it's much easier to reach page 1
Now you concentrate you articles on 10 of those keywords and you ll get better results as if you try to work out the main keyword 😉
finally, spot on a great domain name related to your niche, one of my blog got indexed on page 1 even though I had 3 articles.
A range of articles is key, there are probably hundreds of websites out there with the same articles as you, consider producing unique articles/posts and it will make you stand out 🙂
Clever advertising too through Facebook will help interact with fellow new and old visitors 🙂
Time. It look me seven years from starting my sites to being able to 'quit my day job' to run them. I'm still starting blogs now that don't get much traffic, but it's increasing. You don't get overnight success. You need to build an audience