That distinction could be demonstrated by market maturity, which could be further defined by how large the incumbents are and how saturated the market has become.
Google is a giant now, and has made the search engine market both mature and saturated (along with its competitors). Before Google came along, there were several other search engines, but the market wasn't at all saturated. It was much easier to enter that market with a well placed innovation in searching while the market was primed to grow even larger among a set of competitors who had not dominated it yet. The market was large enough that an innovative company would capture first time users and spread itself organically among them, eventually stealing them from other engines in the process.
Conversely, the search engine market has become saturated. To start a new search engine, you'll need to steal users from an existing search engine, because there is virtually no one who doesn't already use one. As a basic concept, it's extremely difficult to differentiate on the core idea of a search engine. You will not beat Google just by offering a new technical innovation, you'd likely have to beat Google by adding search innovations to a novel platform that is emerging to challenge Google before they capture that platform as well. That sort of combination would offer a new fertile market that hasn't been saturated. As an example, a search engine for applications would have been a legitimate threat, had Google not already developed it for Android and Chrome with Apple competing.