Here are a few reasons why
1. Targeted Audience. With niche specific adsense sites, your target audience is very specific. It is easier to identify who your audience is when you are operating in a niche.
Why does it help to identify your target audience? Several benefits in this
a. You can more easily find put where they are likely to hang out online and go get them from that site to your site.
b. Once you know who they are, you can more easily identify their needs and cater to this. Internet marketing after all is about finding a hungry market and feeding its needs after all.
c. Learn to speak their language. Again, once you have identified these people, you can quickly learn how they communicate, the subtle nuances of the niche and incorporate that into your website content
2. Conquer the niche. It is easier to become an authority online in a specific niche than to become a guru on everything.
It is authority and reputation that internet users buy from.
Imagine visiting a website about this that and the other
And on a page he say's hey 'i totally recommend product x'.
And you look about the site to see what his authority is based on.
What experience does this guy have on this topic to recommend this product?
Only to find no other information on the topic.
You are not likely to buy.
You need to establish yourself as an authority in a particular niche to get repeat sales.
Even the mighty Amazon, started as a bookstore. It is only when they attained authority and reputation that they diversified into what Amazon is today
3. Generate Content For Your Site. Structuring and generating content for a niche specific site is certainly less complicated than a general site. With a niche specific site the structure of your site is almost already predetermined. The requirements of your readers more easily defined and researching content is much easier
4. Search Engine Optimization. This is the biggest reason of all.
Identifying niche specific keywords to optimize your website for is much more straight forward in niche specific sites.
Also getting traffic via your SEO efforts is more successful with niche sites than with general sites.
You see most people when they go onto the internet are looking for the solution to a specific problem.
E.g I am bored and want to play sudoku online [This person types in 'sudoku online' which is a niche specific keyword. He doesn't type in 'I am bored']
5. Website Conversions. Loads of affiliate marketers have reported better conversions on their niche specific sites than general sites.
Now personally, all my websites are very niche specific so I cannot discuss this topic from personal experience.
It however stands to reason that the person typing in 'sudoku online' is more likely to play the game when they get to the site, than the person who typed in 'I am Bored'
It is safe to conclude that conversions either with adsense or affiliate products or your opt in form from niche specific sites will be higher than for general sites