Have you ever been told that? Have you been told you need to put your site in a certain niche?
You will not fail as a site if you do not have a niche, which I at least agree with Nicholas Scalice’s article You Don’t Need A Niche!
What you do need is some type of focus. It does not have to be on one particular niche. You would be surprised the amount of people who claim that their blog is in a certain niche, but covers a lot. For example, Blondish.net is not just a blog about blogging. It covers all the aspects from the design, the content, engaging with the blog community, and promoting the site. Now, what is a niche site is WPAddict.net, which is all about WordPress.
However, this is the site focus. It is basic Web Design 101 where you have to brainstorm what you want your site to do. If you want it to be a niche blog, then go for it. You just need to be a lot more strict with the material you put out if you want to be in that niche.
If you are a person that has many interests, you can have a site that does that, but if you are confusing visitors, you might want to look at what is driving them away and focus on what is attracting them.
However, in the end, you are the one that controls how successful your site becomes because you were the one that put it together. You have to put the leg work in to promote it and engage with others.