Think of it this way.

If something is wanted a large amount by a large number of people, it exists already.

So you’re left with 3 options.

Imagine the graph below:

Large Number of People Want Little		Large Number of People Want a lot
Small Number of People Want Little		Small Number of People Want a lot

So the upper right corner is in red because it exists already.

Now the bottom left (Small number of people wants little) is not worth pursuing so it’s in red also.

You’re left with the top left and bottom right.

Paul Graham says you should always go for the bottom right.

Not all ideas of that type are good startup ideas, but nearly all good startup ideas are of that type.

Paul Graham, How to Get Startup Ideas