Venture capital groups are not just broad market investors that put their money wherever there is a potential cash upside. They specifically look for high risk, high growth industries and opportunities where there is a lot of M&A and IPO activity. They need their investments to become liquid in a short period of time, so companies that are just profitable aren't good enough. They need to be purchased or taken public for them to matter to a venture capital group.
Companies, Not Just Ideas
Every company starts with an idea, but when it comes to writing checks, venture capital groups tend to look for ideas that have already turned into operating companies. The early stages of ideas that are just on the back of a napkin tend to be the domain of Angel Investors. Once the company has gotten some traction and begun to grow, then it becomes the domain of a venture capital group.
Unlike an angel investor, a venture capital group can only invest in deals that have a huge potential upside that will likely lead to an acquisition or IPO.
In order to do this, venture capital groups must target companies going after extremely big markets that have plenty of room for these types of outcomes.
What's considered a "big market" by venture capital standards? The most consistent answer would be a market that's big enough to support a company to go public. Short of being acquired, a company going public is the only outcome that will provide the liquidity that a venture capital needs to get its return on investment.