Major Possible Cause
One of the major possible causes is the presence of tons of sponsored ads Google display on the right hand side as well as the top page of search results.
There are cases where the top sponsored ads could have up to 3 ads at the top and 8 at the right hand side. These sponsored ads are there because other people are advertising and paying money to showcase their business on the same spot as you. The listings on the left hand side are the actual site you are competing against and they got through offsite SEO (link building) and they didn't pay a dime to Google to be listed there.
If you eventually rank your site high for an advertised based competitive keywords, you are bound to lose a lot of traffic and it might be a waste of your time except, of course, the keyword pulls in thousands of thousands of searches each and every month.
My Little Research
I carried out a little research for two keywords that generates about 9, 500 exact searches monthly on a global scale, one on money making niche and the other on health niche. The business keyword has 8 sponsored ads both at the top and right hand page of Google for my business term while the health niche has only one.
The business keyword was on position 2 and the health on position 5 but I was receiving x3 more visitors from health niche on position 5 keywords with minimal sponsored ads than the business keyword on position 2 with 8 sponsored ads.
It is imperative to these days to consider this unprofitable trend to avoid wasting valuable time and resources gunning for just empty air.
So, when you hear the gurus say, if you find a keyword that generates a minimum of 2, 500 global searches monthly, you have a goldmine, sorry, that doesn't include keywords with several sponsored ads like the one I mentioned above.
Verifying this is very simple, simply Google your chosen keyword and check how many ads you have on the top and right hand page of your search result. If they exceed 3, aim for much higher keywords searches otherwise you will be disappointed.
The best spot that literally receives a considerable amount of keyword searches still remain position 1 to 3, this also depends on a number of factors like the niche, amount a keyword generate on a monthly basis.
I hope this saves you from pitfalls that can make you blacklist the internet as a viable source of income.